After giving PD5 and Fusion 3 full evaluations, I'm going back to Fusion for another cycle.
A lot of what I do is testing software in odd developer builds of various BSDs, Linuxes and OpenSolaris; Fusion gives me the least trouble when venturing away from the Microsoft megalith. I have XP and Win7 VMs too; I just don't fire up either more than once or twice a month. I typically have 2-3 other VMs up and on my network at any given time.
I also did take a good look at OpenBox; it's the best of the bunch for running OpenSolaris (wonder why?), but for anything else, particularly the oddball Linuxes, it gave me as much trouble as PD 5. Where Parallels would crash like a drunken teenager in a Maserati, OpenBox just would not install several important systems. So much for that.
VMWare isn't as slick in some areas as PD5 (LOVE that one-click 'exclude from TM' feature, and the VM list is nice), but it DOES have one feature that's absolutely non-negotiable: It tends to work better/more often. That's worth the extra US$10 or so for the upgrade. (with an additional new license for my new MBP).
I've put about 20 hours over the past 2 months into formal evaluation of the hypervisors mentioned (along with a couple of hundred hours of just using them to run VMs with which I was working, and not trying to think "is this better than...?"). The pain level with Parallels has been fairly consistent; VMWare only surprised me a couple of times. Parallels also gives you only half the review time that VMWare does before you have to pony up. That tells me that they aren't as confident in their product (and rightfully so); there are problems that merely suggest themselves in 15 calendar days of varied use that you could really get a handle on in 30. Not having confidence borne of experience in whether I can solve subtle issues as they arise wasn't the only reason for rejecting Parallels Desktop 5, but it certainly didn't help.
The impression I get from using both products, from reading what has been written and from chatting with other users, is very consistent. VMWare portray themselves as a company that tries, and often succeeds, to do things well - starting with a truly usable, stable, versatile virtualisation platform. Parallels, on the other hand... by bringing out releases well before they're really ready, and with a shorter evaluation time, give the impression that they're desperate for your money. Give it to the company that actually earn it.