Because of the speed boost that Parallels gets from running off of a real hard drive partition rather than a virtual one, I'd really prefer to run Vista through Parallels off of my Boot Camp drive. Running it through Parallels will allow me to run many browsers side-by-side which reduces my development time.
There are at least three reasons why I run Vista on my macbook.
a) I'm a developer and need to test on Vista
b) My job requires me to use Windows software
c) My company is upgrading to vista and I prefer to remain in sync and insure the highest level of compatibility with my work environment
I'm running it on boot camp in the hopes that Parallels will support a Vista Boot Camp drive sometime in the future, or Apple will provide its own solution.
For me, Vista in boot camp runs extremely well. I've minimized some of the annoying "features" (without totally compromising security), and adjusted settings to reduce any performance issues (which are more perceived than actual, for instance turning off certain animations speeds things up considerably, but not because the computer couldn't handle the animations, rather the animations were slow and took time to pass).
Aesthetically, I like parts of Vista (many of the button elements, the start bar, etc.) and dislike parts of Vista (the horrible window chrome, the inconsistent UI elements, etc). Of course, Mac OS X has things I like and dislike as well.
