I use Bootcamp but have used Parallels on a mates machine.
I personally prefer Bootcamp (even with the faffing about of rebooting etc). The reason for this is that I don't have anything in Windows that I regularly need to run so can use Mac most of the time and Bootcamp is slightly more stable and faster than Parallels. PC gaming for example pretty much need Bootcamp.
My mate prefers Parallels as most of the time he has various programs that only work in Windows which he needs to use daily and alongside the Mac software that he uses. This means that rebooting etc is too much hassle for him.
Incidentally he also has Bootcamp installed for the times when he wants to run Windows natively for the extra speed and stability (again mainly gaming).