I built a kick-ass machine for a friend yesterday, and my co-worker berated me for not using RAID (and specifically RAID 10). I didn't do it with my DAW primarily because of the cost (but also power consumption/heat/noise and I think it is SERIOUS overkill for R10), and because I backup the main data drive and project drive anyway. And I didn't feel at the time, and still don't now, that there/is was an appreciable performance gain.
 
With 3 separate drives, the risk is mitigated somewhat, though I can see the potential value. But besides time, losing the Operating system drive isn't catastrophic because nothing is on it. I can see a RAID1 potentially replacing the project and sample drives, but still not the OS drive.
 
Let's discuss!

The end purpose of the machine should dictate the build/configuration.
For most DAW scenarios, RAID simply isn't necessary.
For a corporate server, that's a different story.

 I haven't set up an array on this system yet, but I'm considering it. My last system, I had a raid 0 array, same performance boost as raid 10 without the redundancy.
 I'm not really sure what the performance benefit would be today with SSD's with RAID 0 or 10, but it sure helped with HD streaming on my old P4 machine when it came to game load time and sample streaming.
 I'm considering either RAID 1 mirrored or RAID 10 now mostly because I'm using SSD's now and I'm reading a lot about SSD's not lasting as long as conventional hard drives. If a drive is lost on a mirrored array, no need to resort to a backup, just replace the lost drive and rebuild the array and your data is as you left it. Peace of mind backup plan.
http://www.thegeekstuff.com/2010/08/raid-levels-tutorial/
 

If you do regular full hard drive backups, there is no need for RAID.  I had a RAID 1 setup a while back and a Microsoft update corrupted it.  Fortunately with RAID 1 both drives had the same data, but as I do regular backups realistically there was no need for a RAID setup. 
 
As far as SSD's not lasting as long as a conventional hard drive, I have an Intel SSD that is about 1 1/2 years old, that is my Windows 7 boot drive and has Sonar installed on it (but projects are on a different drive).  The Intel SSD Toolbox utility shows it still has 100% of the life remaining.
 
 

With mechanical drives I have had two arrays both RAID 0 just for the performance alone. One array OS and programs the other data and spead the page file between the two arrays. And yes there was a nice performance difference.

With SSD's I use one (drive) for the OS and one for Data no Raid array at all. the performance blows away the spinners using RAID arrays

I might consider using RAID 0 just for the data buts that's about it .

No RAID for me either.
After loads of reading it just didn't seem necessary for my needs.
Actually seemed like it may cause more problems.
 
Mike

I have a pair of small (74GB) Raptor drives in a RAID 0 setup.  I use this for sample content.
 
I got these drives cheap on eBay.  I wouldn't have considered it worthwhile otherwise.  The advantage is that softsynths like SampleTank and Battery load up much quicker.
 
I keep a backup of the sample content on another drive, just in case.

Raid 1 is only if you are lazy (don't back up your OS or data) a simple program like paragon will set auto back up for you
preferably to an ext drive/nas and as mentioned still not the end all.
 
raid 0 has no place in a DAW anymore but does belong in a NLE system
 
raid 10 would be a good idea for live recording of massive track counts, (mission critical) but pointless in a daw for studio use.
its also a poor mans idea of security
 
raid 5, 6 same other than live rigs (again also belongs in a Video system) but at least its the RIGHT way to do raid for security.
it also requires a real raid card (onboard raid is crap anyway other than for raid 0)
 
and even with all this raid stuff if you don't have a separate back up you are an IDIOT and I am speaking from experience.
 

fireberd
If you do regular full hard drive backups, there is no need for RAID.  I had a RAID 1 setup a while back and a Microsoft update corrupted it.  Fortunately with RAID 1 both drives had the same data, but as I do regular backups realistically there was no need for a RAID setup. 
 
As far as SSD's not lasting as long as a conventional hard drive, I have an Intel SSD that is about 1 1/2 years old, that is my Windows 7 boot drive and has Sonar installed on it (but projects are on a different drive).  The Intel SSD Toolbox utility shows it still has 100% of the life remaining.
 
 

 
I have made a few full backups on Blu-ray and a few incremental ones on a backup drive, but I'm starting to think the one thing I'm trying to accomplish by backing up my full drive is to avoid having to reauthorize plugs that rely on the machine code. One of those plugs, Philharmonic, I've used all my authorizations. I'm thinking a restored OS on a new drive is going to change the machine code and I would have to reauthorize anyway? Same with reinstalling a backup to raid. If that is the case, I keep a pretty lean main drive, important data such as project data and purchased plugs get redundant backup to external drive and hard copy. I might as well just do a fresh install and not worry about system backups in case of main drive failure.  
 
Great to hear you've had one running for 1 1/2 years, that gives me a bit of confidence. I have 3 SSD's now and all is well. When I upgraded my father to Win 7 on a Wintec SSD last summer, the installation lasted 3 weeks then the hard drive failed. I'm not sure if he defragged it to death or what happened. I've since learned to choose my SSD's with the same amount of research as I did with conventional drives.

http://www.icydock.com/icy_tip/maximize_intel_ssd_x25-m_performance_in%20RAID_0_for_mac_pc_with_mb982spr-2s.html
 
I have an older version of an IcyDock hardware raid 0 device (well, I keep it set on Raid 0). It is a nice performance boost.
 
In the webpage linked above, they measured a single drive writing at
Write: 82 MB/Second
Read: 240 MB/Second
Practical Write 96kHz 24 bit mono tracks: @300
Practical Read 96kHz 24 bit mono tracks: @873
 
Using Raid 0
Write: 134 MB/Second
Read: 253 MB/Second
Practical Write 96kHz 24 bit mono tracks: @490
Practical Read 96kHz 24 bit mono tracks: @920
 
In Bob Katz's book, Mastering Audio, he has a table that suggests that with these practical recording speeds around 63 MB/Second you can record around 230 Mono 24bit 96kHz tracks. I extrapolated that to get the numbers above.
 
What this really means is that a standard plate hard drive these days which writes around 30-50 MB/Second will probably be able to write more tracks than you will ever need.
 
Basically, hardware RAID 0 with 2 drives is about 160% the speed of a single drive. My personal experience isn't this good. Maybe 140%-150% but my tests are probably not as clean as IcyDock's tests.
 
I looked for some data on Windows 7 Software Raid and found this...
http://www.youtube.com/watch?v=tQiZVoJqyqo
The software RAIDs in the video both end up around 84 MB/S as lows. Pretty solid, but some of the initial speed is caching the work in RAM so I'm not sure at all that the speed is truly sustainable. What rate the soft RAID could sustain over a 3 hour concert (and the lowest rate is what would matter) might drive you down to the range of 300 24bit 96kHz tracks.
 
I don't know if this is very accurate, but it sure is interesting.
 
Best,

Geoff