I have used Parallels for about 3.6 yrs now on both a Macbook and MacMini (Core Solo and Core 2 Duo). The app is amazing. Fusion was good too. I have also tested Win XP Pro in Boot Camp, which is a tad bit better.
However, it depends on what you plan to do. If you need to access MacOS X frequently, Boot Camp will be a pain, because it is not a Dual Boot situation, but rather Reboot to WIN or MAC only. Parallels or Fusion is a Dual boot config as you can use MacOS X and WIN XP (for example) together in real time at virtually real time speeds; MacOS X will be you main OS and WIN XP would be the 'guest' OS.
For me, I use many Mac OS X apps and a few WIN apps. There are a few WIN apps that I absolutely need to use for Biz, but Parallels help me solve that issue seemlessly. Hope that helps.