I got my Mac Pro around 8 days ago, and it has been pretty awesome. I've been on vacation for that time, so I have been using it for probably at least 12 hours a day (Gotta love vacation). It has been flawless. I have had no crashes or issues. I have added an IDE hard drive and an external USB HD. The stock Seagate hard drive is a bit slow, but no slower than any of my other hard drives laying around, so it will hold me over until I get a Spinpoint F1. If I could do it over, I would have gotten the 320 and just bought another HD.
I have:
2x 2.8ghz
2GB Ram
500GB HD
8800GT
Wifi
I have downloaded well over 100 gigs of stuff, and played my fair share of Crysis, COD 4, and now Bioshock (which is a pretty crappy game, imho) and the Mac Pro runs them all beautifully. I can't even begin to slow it down with processor intensive tasks. This is, without a doubt, the fastest computer I have ever used.
In all honesty, I was a little shocked I couldn't push it further. Even when I throw everything I have at it, I can barely get the CPU usage above 60%.