The XP x64 system is actually quite stable - I've got no complaints there. What I have had trouble with is running a variety of software packages and installing some drivers. A lot of them are network management utilities and some older (but useful) apps. Initially I couldn't even run our corporate antivirus package, although a lot of viri wouldn't run on x64 either
I also have some older hardware devices that just won't work and never will work on x64 so I keep an x86 box around for that.
Over time, I have found more and more applications and hardware devices are catching up. x64 is getting to be very usable but there are a lot of things that don't and never will work with it. Your millage may vary.