Parallels experience
I have a Mac Pro (2x3GHz Xeon processors, 4GB 800MHz memory), and Parallels regularly takes 20-30 minutes to start and boot Windows 7, and then runs so slowly a lot of the time that it's completely unusable. I did manage to improve things by disabling all of the sharing of folders, desktop etc, but it's still seriously slow. I previously tried Fusion, but had some issues with it too. On my MacBook, fusion worked acceptably, but I still had issues with Parallels.
But one thing that really makes me want to give Parallels another shot is that they have excellent customer support. They spent a *lot* of time working through issues with me in the previous version to get it running, and have been really good to deal with. I have tried to use the Fusion support, and it sucks big time. Feels like a huge corporation who just don't care at all. I also visited the Fusion stand at a show in London, and asked some questions, and they tried to just avoid the questions and give some marketing bs saying it was wonderful and they were sure it would do whatever I wanted. Very not happy...
The only thing that is holding me back from upgrading to Parallels 6 is the upgrade price (and confirming that it fixes the performance issues). My expectation for software is that upgrade prices should be somewhere about 25-30% of the original purchase price. $50 for the upgrade of a product that retails for $80 just seems ridiculous.
Fusion's $10 offer sounds appealing, but my experience with their support was so bad that I just *never* want to do business with them again, period.