Parallels Desktop 4.0 came out today, and although 3.0 was working fine for me, I decided to upgrade right away after reading some initial reactions. At $40, it didn't strike me as a horrible expense.
The first thing that sold me was that it finally supported multiple CPU's. My Mac Pro has four, so naturally I thought it'd be nice to give my Windows XP VM a little more processing power. There's actually a setting now to let the app manage resources for you, but I'm not sure I entirely trust that. Regardless, the usual combination of Visual Studio, SQL Server and such is visibly a little more snappy. I notice ReSharper Intellisense acting faster in particular.
With the initial reports of some speed differences, I admit that I had a silly desire and hope... that RollerCoaster Tycoon 2, in all of its 2002, sprite based assembler glory, would run more smoothly. Before, it was very choppy, almost to the point of not being fun. But what do you know, it runs better than it ever did on my PC's of that era. I'm pleased.
Apparently the upgrade can potentially hose shared Boot Camp/Parallels partitions, but I don't have that setup. So I'm pleased with the little tweaks. The UI is prettier too. The whole product feels more polished.