The biggest problem people have with x64 windows is driver availability. The major manufacturers provide 64-bit drivers now, most of them now getting out of beta. I personally run the 64 bit of windows on my desktop, I would run it on my notebook if it was 64-bit capatible, and given my hardware is from major manufaturer's there is a few versions of 64-bit drivers to choose from. The only beta drivers I am running are for the Creative X-Fi and the nForce 3 250 chipset, both of which are completely stable.
The only program/game I have not been able to get to work is Splinter Cell Chaos Theory but this is a known issue with Ubisoft but they are not doing anything for it. Eh, oh well.
I have read people experience performance losses with 32-bit programs in x64 windows, I have not experienced any losses.