Thanks all for sharing your thoughts.  I'm trying out a USB mixer right now, a Behringer unit, that operates really well at 19.5ms RT as measured (it reports 22.4).  The way it's setup I can direct or live monitor as I'm recording - I gotta tell ya, I don't feel any difference.  With Record Latency Adjustment set to the measured number of samples, everything lines up perfectly.  Maybe it feels OK because the vast majority of the latency is on the output (~82%+ of it).  If my thinking is correct, all of the audio I'm playing to is experiencing this, so in effect it's the input latency that matters most for feel; 3-4ms in this particular case. 
 
I like this unit because I need to replace my old, dead mixer anyway.  I can always add a lower-latency interface later if latency becomes a problem.
 
It's a shame about my Delta66 and Win8.1.  The big problem now is that even though the Delta may be working perfectly, it will always be the scapegoat for any problems with Sonar that come along.  The Easy Way Out, if you will.  I wonder if Avid *really* thinks that everybody they force to abandon Delta series interfaces will end up buying one of their others.

Avid/M-Audio's driver support for new releases of Windows became so poor that I stopped buying anything by them two or three years ago. A pity really, the Delta series were very good value and being PCI gave great latency times compared with most USB interfaces at the same sort of market point.

Depends on you - I can play guitar and keyboard (near) perfectly with a 1024 buffer (as in timing wise, the notes I play might not be right :-)) but one of my bandmates can't record his vocals with anything more than hardware-based ("zero") latency.

I'm not sure people understand 100% about latency. 
First, there is a separation between recording with MIDI and recording audio tracks. 
 
I can have the LP64 multiband compressor on the master buss and record audio without issue. 
I have to bypass it when recording midi. There is huge latency. Why? 
 
Input echo. Monitoring your signal path correctly. 
 
We have to use input echo to record midi using a soft synth or we won't hear anything. It is well known that "look ahead" plug ins cannot be "on' because of the latency they add. The midi information arrives at the soft synth within 1-4ms. But the processing of the DAW now slows everything to a crawl before it makes it back out to your monitors. So we need to minimize processing and have low output latency. 
 
We do not have to use input echo to record Audio. We monitor at the interface. They miss name this feature "Zero latency"  to make you think your $100 interface is awesome. 
Of course it's almost zero. But it's got little to do with round trip latency. It's got a lot more to do with how Sonar and the audio drivers calculate the interfaces latency. 
 
Your listening to your playback that Sonar has politely adjusted to sync with the incoming audio signal. It is actually playing the DAW output "ahead" of time so this will happen. 
So your playing to a time aligned playback signal along with your performance in real time at the interfaces analog headphone and output stage. Therefore the term Zero latency monitoring. 
  
Even using the worlds worst performing RTL interface you will hear no latency or very, very little as long as that input echo is off.  
Turn on input echo and you will now hear your RTL as a sort of digital delay. 
Mess with buffers etc and this can be minimized to almost acceptable levels with the right interface. Myself, I'd rather use the direct monitoring. I can't take much more than 5ms and it bothers me. My systems can't get below 12ms. Too much. 

johnnyV

 I noticed that ESI cards rate very good and are under $200. The Julia xTe is a PCIe unit and it is up there with units 4x the price. 

johnnyv...how can I use direct monitoring with MOTU 8Pre...and correct me if I am wrong, but direct monitoring would not allow me to hear TH2 processing, so some other means of tone for my guitar would be needed to allow playing the parts with the correct attitude, if you know what I mean.