I guess I'll throw my 2 cents in also. I purchased Parallels
before I bought my iMac because I'd demo'ed it at an Apple store and was very impressed with it. Up until then, I didn't even know you could run Windows on a Mac. After hanging around these forums for a while and reading the "which one is better?" threads, I started seeing LOTS of posts endorsing Fusion. So last night, I installed the demo. I'm not using a Boot Camp partition (for either Parallels or Fusion). I'm using the same copy of Windows XP; I just haven't activated it yet. I'll do that when I make a final decision.
My initial impression of Fusion is that I like the interface; it looks like a Cocoa application ("Mac look and feel"). One thing *really* like is the ability to hide the Windows task bar and launch applications from the OS X menu bar. I think it needs some refinement so that the menu tree can be rearanged so that folders can be listed before or after individual programs. Maybe that's already possible. I don't know....
I think Parallels is a little more customizable. It might seem silly, but I prefer the way the screen transitions to and from Full Screen mode in Parallels (the rotating cube) as opposed to the "dissolve" mode in Fusion. I also like the way you can choose
how it transitions (cube, flip, etc.). Granted, all these things are relatively unimportant compared to how well Windows programs run within the virtual environment. In this respect, I think they're about even.
My biggest disappointment with Parallels is their support. I've said it before and I'll say it again: the Parallels support forum is where questions go to die. While there are some threads that get a lot of attention, many (most?) just languish for a long time with many views and no responses. The last time I had a problem, I posted detailed a detailed description of the issue and we're now up to 104 views and no responses. It kinda leaves a sour taste in your mouth, knowwhatimean?
Anyway, after all that rambling, my advice would be to download the demo for each, install each without using your Boot Camp partition (this way they remain separate files), use the same copy of Windows for each (you can use it for 30 days before activation is required); install some light programs and just play around with each to see which you prefer. Sorry for being so verbose.
James