I have a new quad 6700 with Windows 7 64 beta and Ubuntu 64 that I am testing out for future installs for clients. Ubuntu is just as slick, performs better and costs less. Going to be a steep hill from here for MS.
In both cases, I think the 32 bit versions run better though. 64 bitters can use more refinement.
I've been running Ubuntu 64-bit on my desktop for about a year now with few problems.
But then I chose my hardware carefully. If you build a box using hardware that relies on manufacturers to provide your drivers then you can have issues with 64-bit. People with wireless cards with the Atheros chips have had lots of problems. I use cards with the fully documented RALink chips and haven't had any problem at all.
The last stumbling blocks were 64-bit flash and a 64-bit java plugin for Firefox. Both of those finally showed up late last year.