I have tried both Fusion and Parallels, for both XP and Vista. I did not care for Fusion at all, maybe I didn't give it much of a try, but I did not care for the look and feel and it wasn't as seamless as Parallels. Parallels on the other hand did not get in my way, I use Coherent View and XP stays out of my way till I need it. No need to use two clicks to open something, like using the Start menu, it runs just as it would if I was on a PC. So, no need to use one click to activate that window and then another click to use, doesn't sound like a big deal, but gets a little annoying having to make windows active. Clicking from window to the other.
Bootcamp on the other hand I have not used. I'm not sure why it's needed over a virtualization situation. There are really only two scenarios that I could see that would have me to need to use Bootcamp and have to reboot to use Windows.
1) I work in an environment where I use Windows exclusively and am allowed to use my personal Mac, so I would just use Bootcamp and reboot into Windows for the day.
2) There is an issue concerning ram like with a MacBook, where I have something like 1 GB of ram and want to also run Vista, then I could use Bootcamp to be able to utilize all the ram, trying to use Vista on something like 512 MB of ram would be torture.
I prefer using XP over Vista in a virtualization situation because for one, Vista still doesn't run everything I need, my University has software that I need that only runs on Windows and doesn't run on Vista very well, not at all actually. And XP runs just fine on 512 Mb of ram, and if you have ram to spare it runs better then a PC.