I've used both, but am now using VMware. I used Parallels for almost 3 years, it was good. But I did have a few support issues where trying to get help was a pain. Once I had a corruption issue with an expanding disk drive image that could not be recovered. Another time an upgrade of Parallels somehow messed up the default applications under OS X.
The last issue was where right clicking on a file in Windows was causing a delay while my external RAID array had to wake up the hard drives. THis caused a 20 second delay many times a day. This problem only arose after an upgrade. The RAID array had nothing to do with anything in Windows, but it drove me nuts! (Ok, I was already nuts)
The tech support folks were all Russian, there was a language barrier there, they never understood what I was talking about. I was never able to resolve this.
I switched to VMware about a month ago when I got my new Mac Pro.
If you run it in bootcamp, you won't have any problems since it's not virtualization. From what I've read, macs run windows better than windows pcs. I have not been especially impressed with any virtualization software, I agree.