Hi Beepster, just thought I'd update you on my progress...   
 
I've had the 18i6 running since Wednesday evening, and except for a glitch in the first hour of setup it's been running well.   Here's what happened - After getting the drivers and MixControl installed and updated, SONAR recognized everything just fine.  I then tried the Hi-Z input with bass and guitar to see how that faired... about 10 minutes in I suddenly lost the connection to the 18i6, both in SONAR and MixControl.   I tried unplugging and replugging the USB cable, rebooting a few times, etc, but none of that worked.  The only thing that did work was cycling the power to the unit.   But I've now had the 18i6 running constantly since them without any further hiccups.  Whew.
 
I did notice a bit of noise when turning the Input gain and/or Headphone volume way up, but since I would never have either of them quite that high in regular use I wasn't overly concerned.  At least not yet.  The Hi-Z input does sound pretty good to my ears, fairly thick and present.  Definitely better than my old Roland VM3100 and about the same as the Lexicon Ionix 422 (which was surprisingly good considering).  I haven't tried any of the other inputs yet, but I am eager to try a mic - Focusrite has a good reputation with preamps.
 
For latency, the lowest setting (1ms on the slider) gives me about 3ms input and 4.1ms output latency, for a total of 7.1ms roundtrip.   That's a good 5ms better than the Ionix' lowest settings.  I was able to load up Rapture, Novation Bass Station (love it) and Dimension Pro at that latency and just squeek by.  Once I added Zeta2 I had to increase latency to the next value, which gave me about 5ms input latency.  This was still very good for live playing and there wasn't a crackle or pop in sight.   Again, better driver performance than the Ionix.
 
I also loaded a fairly full project which previously forced me to use the Ionix' maximum latency setting (2048).   With the 18i6, about half way up the slider seemed work just fine.   And where the 50% CPU range would mean increasing latency with the Ionix, it's now about 70-75% with the 18i6.   And that extra 20% or so is very welcome in my book!
 
So far, so good...
 

That's great and thanks for letting me know. It's weird it gacked out like that on you though. Maybe something wasn't working at full capacity on the initial start up and need a power down/power up to get to normal operation. I unplug mine every time I'm done with it so I guess I avoided that issue if it's a regular occurrence. I haven't noticed any hiss or anything yet but I'll keep my ears on it. I generally have the headphones turned up pretty loud because I find that output to be sufficient but a little on the quiet side. Probably a safety feature to keep people from blowing out their eardrums. Anyway, have fun with it. I know I've been enjoying mine immensely. Cheers!

Anyway, have fun with it. I know I've been enjoying mine immensely. Cheers!

 
Ditto!  As long as it continues to work as it has over the last 48 hours, I'm good. 
 
Of course... I also purchased a BCF2000 so my time right now is split between the two.  Fun stuff. 

Well that looks like a useful little dealymadoodle. Eventually I'll have to look into control surfaces like that. Any extra cash has to go towards other stuff for now though. Like GR5 (maybe) and an Axiom 25... I should probably buy some groceries too (so hungy). ;-p

Plugged in one of my low end condensers tonight. The sound was ridiculously clear. In fact it brought the mic out so much I noticed the true characteristics of it. I need a new mic. lol

Beep, I've been following your post since it's inception, I've been using a Line 6 UX-1 for about 4 years now and it works..........But there is the "What am I missing out on" factor that wants me to get a new interface. After your weeks of extensive and vigorous field testing................Do you recommend the scarlett 18i6? The price point is another decision maker. It's a toss up between the Quad capture and the 18i6. (but the 18i6 has more input options.)  I'm flip floppin like a flounder on the beach and I cant decide. 

Hi, keneds. The short answer would be yes. I think it's a great little box so far and I am quite happy I chose it.

The long answer (there always seems to be a looong answer with me doesn't there? lol) is, probably. I have only really been doing some very basic stuff with it so far. Who knows, the thing might die or catch fire or call me nasty names when I'm not around after a while but I haven't seen any reports of malfunctions online so that's probably unlikely. The other thing is the Quad Capture is supposed to be the superior device of the two in regards to latency and signal quality but not having a QC to compare it to I can't say for certain. I just took a quick look at the QC on the Roland site and it does not seem to have an optical/ADAT connector for expanding the inputs which I thought Seth had told me it did but he must have meant another unit. In fact it doesn't seem to have much of anything. There have also been reports of some of the Roland i/o drivers being a little wonky but that might be for older devices and I'm sure once they release the next round of drivers those issues will be solved. But IMO unless you REALLY need the ultra low latency I don't think that unit is a very thrifty decision at $270 (the 18i6 is $300). Maybe take a look at the other Roland stuff that does have the expansion options. I say this because if you are worried about inputs expansion is one way around it. Just use the unit as is for light everyday tracking and if you need to record a full band at some point you could probably rent a mic pre box. The Scarlett however DOES have one of these connectors which is cool. Also if your old interface has an optical OUT (it has to be out) you can supposedly use that as an expansion unit with the Scarlett. I'm not sure how well this works but there are a couple other people on here who've said they've done it with the Scarlett and Matt from Focusrite told me that it was indeed that simple. I'm gonna try it with my Layla 3g at some point to bring up my multi in/mic pre count to 4. Speaking of which one other thing to know about the 18i6 is if you intend to use more than the two multi ins on the Scarlett be aware that the other inputs are line ins only so you'll need a mixer, DI boxes or amps with line outs on them (I'm pretty sure that's how that works) and without an expansion box or using SPDIF connectors you only get 8 inputs. But that's enough to record a full drum kit or do bed tracks with a full band which is what pushed me over the top to buy it.

Anyway, if you read the thread and have been looking at other reviews and articles online about the 18i6 you probably know most of that stuff already but I figured I'd condense it down in one post. Basically if you can tolerate only getting about 12ms round trip latency (7ms for very low intensity tasks from stevec's description) I'd say it's a winner and the price can't be beat for what you get. The high Z sounds great. The mic pres sound great. The MixControl software is super easy to use and allows you do do all sorts of neat stuff. You get a bunch of plug ins as well as Ableton Live Lite and Novation BassStation and the customer support is awesome. The only complaints I have ever seen about this unit were in regards to their old drivers and that issue has been corrected as of the last couple driver/firmware updates which can be found on the Focusrite website.

Hope that helps. Cheers!

Hiya Beep and keneds... regarding the Quad Capture... I'm using one of those, as I have probably said before (so sorry for repeating that). Regarding the so called "ultra low latency," I'm not too sure about that to be honest, although I'm still getting my head around the settings. Some of this stuff is new to me. My old desktop had an internal RME sound card and I don't remember how I went about setting that up.

Anyway, my driver settings are currently:-

Audio Buffer Size = 4 (not sure what that means and it can be varied between 1 to 9)

ASIO Setting = 96 samples (44100 Hz)

and my X1 audio page is showing a total roundtrip of 11.9 milliseconds

I'm not even sure if that is good, but when I tried to lower the Audio Buffer Size last week, my laptop crashed, so I have left it set to 4.

So, as I said, I'm not sure that the Quad Capture a bit better than Beeps 18i6, as far as latency is concerned.

So, that leaves inputs, and it is here, as has been mentioned, that the 18i6 scores. If inputs are an issue, then the 18i6 is the way to go.

The Quad Capture does have coax SPDIF input and output, and you could add, if you wish, another two line/mic inputs, giving you a total of four. That would allow drums to be recorded with a simple pair of overheads, plus a bass drum mic and a mic on the snare. I have an old Roland MMP-2 mic preamp that has coax SPDIF connectivity, but I have not got around to connecting it to my QC to see if it gives me those two extra mic/line inputs.

Okay, perhaps not much help there, and I still need to do some proper work with my QC (I'm still getting to grips with my new laptop/DAW setup at the moment).

Hi, synkrotron. Thanks for the input. It sounds to me like you are actually getting quite good latency from the QC. The limitation sounds to be the laptop. That 1-9 setting is likely similar to the settings on my Scarlett (1-10 milliseconds and no I don't really know how that works into the final equation but that's how the 18i6 and MixControl software deals with buffer settings). So at a setting of what I am assuming is 4ms you are getting better roundtrip latency than I am with my setting at 2. If you had it on a more powerful desktop that could handle the extra CPU load at the 2ms setting it is likely you'd be getting something around 5-6ms roundtrip and at the 1ms setting you could be getting the 2-3 ms roundtrip latency that device seems to be capable of from hearing the Cakewalk/Roland dudes talk about it. I think it really depends on the system at that point. I could however be completely wrong (I usually am about the really fancy tech stuff... lol) but if you know someone with a super high powered desktop it might be interesting to check out. I myself am curious what kind of results you'll get from the QC in general (mic pres, high Z, line in signals, general mayhem, etc) so let us know how it's working out. Cheers!

Keneds - I have a Line 6 KB37 and a Scarlett 18i6. I use them both extensively. I prefer the KB37 for all my electric guitar tracks and working with Guitar Rig 5 and Pod Farm. The guitars while inputting dry onto the track via the KB37 have full on effects applied and sound like I'm playing straight out of an amp. Really natural sound and feeling. And of course it's handy to have the keyboard there when I feel like using that device for an input. That all said, I use the Scarlett for everything else because it works flawlessly, sounds crystal clear, and I can throw tons of inputs at it including through the ADAT input which works fantastic. The mic pres are great and the mix Control software works well with very little to learn to make it go. Sonar X1 is very convenient for me to switch between the two audio interfaces as needed.