Well, I've been using 64Bit since last summer and the only problems i have with it is that there arent any 64bit compitable flash players out there, so i cant view some sites. also i have trouble to find X800 drivers, there arent any real out there so i am using some custom made, not sure if they are official or not, anyway they work fine and i get good graphics out of my games.
there seems to be roumors about bugs but thats just roumors, i haven't noticed any bugs for these 8 months.
although very few applications use 64bit to get preformance of it so I don't think it is much different from 32bit. windows bots fast and it is better at juggling around with multiple applications.