But if you read that "review", you'll see that the reviewer hasn't even used Parallels before, and is seriously mistaken in everything he says about VirtualBox, and doesn't mention speed at all (although the comments below it confirm that VB outperforms the other two, which is my experience too). The only thing VB can't do that Parallels can is run the Bootcamp partition virtually, which is neat, but since Parallels' website wouldn't even let me pay for it I don't rate the support much, again confirmed by other comments.
None of the virtualisation systems will let you run 3d games in a window on a mac, of course, the performance just isn't there. If you want to do this you need Bootcamp. For most other apps, though, virtualisation is fine.
Oh, whadya know, I must have linked the wrong thing. The one I had read before did several benchmarks, various 3D games, tasks in applications like photoshop and office, file copy and hard disk benchmarks within the VM, etc. etc. I've read several where, in actual benchmark tests, VirtualBox rates consistently slower than Parallels or VMWare. (I'm not knockin' it, I love VirtualBox and do use it on other machines, but it is what it is).
For office tasks, VirtualBox is the way to go. Why pay for something you don't need. But, that's just one of several sources that have shown that VMWare Fusion and Parallels run applications much much faster than VirtualBox. If that matters to the user (and they are willing to pay for it), it's worth mentioning.
FWIW, I'm able to run PC games inside Parallels just fine. Runs just as well as you'd expect an HD4000 laptop to run. I had two issues with VirtualBox. One, was it performed poorly in some games and 'heavier' apps. Now, granted, a virtual machine is not the ideal situation to be doing that stuff. But, I really wanted to avoid boot camp, so being able to is a plus. Second, the unity mode wasn't near as nice as coherence, and I had some issues setting it up. The biggest issue, was it never seemed to like to behave when I switched between my 2560x1440 display, and my onboard MacBook Pro display. Parallels isn't bothered at all by it. An experienced user could probably fix both of those things, save money, and chuckle at me over here paying for Parallels. But, for me, with the education pricing, it was worth the seamlessness, performance, and ease of setup and the fact that it worked with all of my 'stuff' quite well.