In order to put Parallels Desktop 6 through its paces, I installed several virtual computers running a number of Guest operating systems: Windows XP, Windows 7, Ubuntu Linux, and even an early version of Chrome OS. When creating a virtual computer in Parallels, you define the amount of resources you want that computer to have. For example, how much system memory should it use, and how big should its hard disk be. Depending on whether you're going to be using your Mac for anything while the virtual computer is running, you can scale this quite high and devote the majority of your physical computer's resources to the virtual one.
I found that if I wanted to run intensive applications like Microsoft Visual Studio, I needed to allocate plenty of memory for the virtual computer. I'm pleased to report the system is remarkably useable even in intensive scenarios like this, and it's quite bizarre seeing a Microsoft specific application running on your Mac as if it's a normal, native application!
I also connected to a number of those operating systems with the iOS application, and it works well. It was a little sluggish on my network, and thanks to the non-touch design of most desktop operating systems, I couldn't see myself untethering from my desk and doing my day job using it, but it's certainly a nice touch and could have its uses for referring back to things on a virtual computer in the absence of a real physical computer you can connect to. In particular office type applications could benefit from this approach.
One thing I didn't test were the graphics performance claims. Most of the games I had to hand are quite graphically intensive, and my MacBook isn't exactly a graphical powerhouse at the best of times. There are a number of articles online that explore the graphics performance in detail though, and the results on the right hardware are very impressive.
If you need to access Windows or Linux specific applications from your Mac computer, or have an interest in alternative operating systems but don't want to be constantly installing and uninstalling to check out new versions, Parallels is a worthy purchase. It's nicely presented, easy to get to grips with, performant and does most of what you'd want without running into any problems.
Parallels RRP is $79.99, and a 15-day demo can be downloaded from the Parallels web site.