I've been using Parellels for awhile, but my month-long trial ran out last week. I was about to go ahead and make the purchase when my friend suggested I check out VMware Fusion, something that I had heard of a few times before but had recently forgotten about it.
I don't have a lot of needs; the only thing I really use virtualization software for is for IE6/7 and just for checking out layouts in the Windows version of Firefox, Opera, what have you. So basically for web development. I virtualize Windows XP for that, but I also virtualize a copy of Ubuntu, too. That's just for the heck of it. I haven't had too much time yet to actually sit down and check out the front end of Ubuntu, but so far I've really been pleasantly surprised by it. If I weren't so enamored with OS X, I'd probably switch to Ubuntu in a heartbeat.
So I've spent a month or two of irregular usage of Parallels, and tonight I went ahead and picked up VMware's offering. Fusion is pretty nifty, something which I'm pretty surprised at. Paralells tends to take all the glory; I suppose it's the first to market aspect of their product, but Fusion is impressive. I admit I haven't spent a lot of time working in Fusion yet, but from my first look it appears to be slightly easier to work with. It's not a monumental work of UI or anything, but their way of doing things just happens to be a little bit smoother than the Parallels way of doing things, in my opinion. XP setup worked a little bit smoother for me on Fusion, and I've had less glitches with Fusion thus far. With Parallels I ran into a few glitches with starting and stopping virtual machines, that sort of thing. Not to mention my Ubuntu virtual machine just up and stopped working one day, forcing me to reinstall (I hadn't made a backup of the virtual machine, unfortunately).
Fusion doesn't have coherence mode like Parallels does, but again, I've found the Parallels implementation to be either slightly buggy or non-intuitive. Besides, I've also found that when I want to work in another OS, I either can work in a small floating window just to get a couple quick things done, or I'd rather just go full-screen mode.
And, as the final kicker... games. I haven't really played any computer games in quite awhile, mostly because I work nearly solely on a Mac. Sure, I play my Wii and occasionally bust out Halo 2, but I do miss playing Day of Defeat, Counter-Strike, what have you. Right now I'm downloading DoD... hopefully that will run smoothly. If it can... whew, that'd be sick. The fact that you can virtualize an entire OS and then run 3d games in it is pretty amazing. Unfortunately they're stuck on DirectX 7 or so, so when I tried to run Stronghold Legends I ran into problems. Hopefully in the near-future they'll be able to improve on that. And if that's the case, I might have to start looking into getting a more powerful desktop, as my MacBook might have problems with its graphical capabilities.