I'm running Vista x64 on my laptop. I'd highly recommend it, for applications that are coded for 64bits you'll notice a solid increase in speed.
If you're going to go x64 make sure you:
Have at least 4gb of ram
x64 uses more ram, but allows you to use more than 3.5gb
Have checked to make sure all of the applications you need to use work.
I haven't run into any problems this way, but my dad who uses a lot of PCB and CAD programs found that some apps he used didn't support x64 and he had to switch back to x86.
Overall, if you have 4gb ram+ and all the apps you need work on x64 then I would definitely recommend it. x64 is the future so you'll see more and more optimized programs so you'll see definite performance gains in x64.
"I'd marry skiing, but it's playing hard to get"