I'm the type of guy that hates change. I used Windows 2000 until '06 when my copy finally quit working due to numerous re-installs (I hated solving problems and would just format whenever something came up). I learned to love XP and used it until about a month ago when I got a x64 system. I was gonna switch to XP64, but heard the driver support was terrible, especially for gaming. I read one x64 comparison between XP, Vista, and Windows 7 and the reviewers couldn't even get XP 64 stable enough to complete the test. But anyway, if your using it for business I guess that's not a big deal for you.
I've been using Windows 7 a couple of months now, and have to say I really like it. It's a little more nannyish than XP, but it looks cool, and does streamline alot of things. I have 4 monitors connected to my PC, and getting them all to work in XP took hours. To my surprise, all 4 sprang to life halfway through the installation of Windows 7. I've never used Vista, but I haven't had any problems when streaming videos or music. I stream movies to other PCs and music to my iphone all the time with no problems.
In my opinion, if XP64 is working for you, stick with it, but Windows 7 is cool to.