I've really enjoyed using virtualization on my computers, both when I was running Windows machines, and then when I switched to Macs.
When I first switched to a Mac I did have to give up my favorite virtualization software from VMWare, and start using Parallels. My experience with Parallels was less than endearing, and when I found out that VMWare was coming out with a version for my Mac I was overjoyed.
I've discussed various versions of both Parallels and Fusion, and I always leaned towards VMWare. The performance to me always seemed much better, and the graphics compatibility has also been better. I've recently finally removed Parallels (I actually purchased both versions) and am using Fusion only now.
Version 2.0 of Fusion is even better, the interface is much more 'Mac' like and easier to use, and it's just cemented my appreciation for this application. I've even taken to doing presentations on my Macbook for Windows Development conferences, and if people can't see my laptop, they never know that I'm running a Mac.
I just read an article where someone compared the two applications side by side, and it basically reinforces my perceptions that VMWare is definitely the better performer of the two products. I'll be really interested to see his next article after he compares VMWare to VirtualBox.