I recently upgraded from Parallels Desktop 4 to 6, after doubting for a long time that it would do anything special, but I have to say that the difference is enormous between version 4 and version 6 !
Using Parallels 6 on my Macbook Pro and with 8GB of RAM both Windows XP and OS X run admirably well side by side. Cutover between one and the other (XP is running in a window) is instantaneous.
I even went so far to play Modern Warfare 2 (via Steam on XP) and it ran, although I couldn’t crank up to the maximum resolution and it ran a bit stuttery when going through a large combat scene. But still, it was playable and it impressed me enormously.
One game I’ve always wanted to play but never got to decently start is The Witcher, and I’ve installed it, and it runs beautifully, with no apparent slowdowns.