I switched to x64 (vista) when I got my current rig about 2.5 years ago. I've been very happy with it. Until just recently, there were a couple of apps that had unsigned drivers
that required me to run in test mode and use an app for "custom" signing drivers. Still, I felt it was worth it just to be able to use all of my 4 GB of ram.
Just the other day, the last of those apps that had unsigned drivers got an official signed driver
and now finally, everything I want to run, runs on x64 without having to use any work arounds. So finally, I was able to stop running in "test mode." So for me, finally x64 has become completely mainstream.