I setup a dual boot XP-Vista installation to see how I liked it. I have to say that I haven't had to boot into XP yet, and I can't say enough positive things about it. It's like driving a 5-yr old Ford with many mileages and wanting all the bells and whistles of a new Chevy. Well, Ford finally has those bells and whistles now and it just drives beautifully.
I love the search functionality, the built-in photo viewer is one of the best photo viewers I've seen... it's quick and I love how many pictures you can see at a time with simple mouseovers to view, autosaving changes, etc; the menu system and default directory structure (C:\Users\<Folders>) seems just right but with search you barely need it, the Aero interface... my wife actually who's not a savvy technical person actually said it just feels "smoother" and "more right." Everything really does feel smooth, responsive and much improved... and "new." I even have not had that immediate knee-jerk reaction to install firefox right away as IE7 fits so nicely with everything (I will be installing firefox still though as I'm tired of getting blocked/prompted for every download). I also like the "hibernate" feature by default for desktops. Save power and it comes up right away from this. Also, for any programs not compatible with Aero, it switches virtually instantly from being off automatically when program starts to returning after program ends. As for performance, the Win-Tab thing works really well. I loaded up WoW, a Movie, two IE7 windows and 4 windows explorer windows and could cycle through no problem.
Also, I only had to change two things to get WoW to work as it did previously. These are well documented on the forums... but disable vsynch and disable some TCP/IP setting network adapter that Vista enables by default. After that it ran just as smooth as before with no hiccups.
On the negative, I have already turned off the User Access bit (go to black and prompt for every bleaming thing). In addition, Media Center does not seem that great. It has crashed a few times and the photo album and slide shows in this are no where near the same quality as that built into Vista by default. Also, if you want to dual boot, don't start the disk for installation within XP... boot to the disk. I believe, this will ensure that Vista is installed as drive C: just like XP (transparent to the two OS's).. my Vista is on E:

But the dual boot set itself up automatically with no changes to any bootloader files or config files.
POSSIBLE BIAS NOTE: Mind you I didn't have to pay to try Vista out. I got a free trial version of ultimate for attending a MS event, but I will be purchasing at the end of my trial. More for me: I support and purchase Macs, Unix, and Windows machines everyday. Most of my work machines (100's) are Mac, however, my primary desktop machine is Windows XP (until company purchases Vista).