I used VMware at my last job to run Windows NT, XP, 7, and Centos 6. It works very well. Seamless integration, excellent speed. Features like drag and drop sharing, shared clipboards and folders, and the like all worked well. Only gripe is that one usually has to do a paid upgrade to VMware if they do a major OSX update. They don't seem to care much about maintaining compatibility IMHO.
At my current job I've used Virtualbox to run WinXP. The VM works fine, although I cannot get drag and drop sharing to work at all - the VM crashes whenever I try. Printing had some growing pains, too. It's also noticeably slower to suspend/restart a VM. That being said, it does the basic job and is free.
If you have the money, I strongly recommend VMware. Haven't used Parallels in many years so I can't comment on that.