I've been using vista 64 for quite a while now and have had no significant problems doing so. I wouldn't hesitate to recommend it, but unless you have a specific need, I wouldn't upgrade just to upgrade. About the only things I'm aware of that it will buy you are the ability to use more RAM (XP 64 is junk), and MUCH better handling of D3D apps across multiple monitors [both of which are important to me].

One thing that has plague me since upgrading: About 1/3 of the time when I exit wow, it will just hang. I actually think this is a video card driver issue, but it's particular to 64 as it never did this in 32 version. It's not a big deal, just click the X, but it gets a little old.