I have a 2.4 GHz Intel Core Duo Macbook Pro with 4 GB of RAM. I do some .NET development using VM Fusion/XP/Visual Studio 2005, and have allocated 1 GB of RAM for the virtual machine. It works fine for me, and I have been happy with its performance and responsiveness.
The only real annoyance for me is that by default some of the function keys trigger Mac events, and can't be used as keyboard shortcuts for step over/into/continue debugging functions. For example, F10 triggers the expose function. However, as @Crash points out, the mac keyboard shortcuts can be disabled in the vmware preferences. This works like a charm - thanks for the tip!
@Soeren Kuklau: Thanks for your suggestion, but I don't think I was clear about my problem. I've already configured the "use standard function keys" option. What I was referring to is that by default, F10 and F11 trigger expose actions. And that's my real annoyance: to use keyboard shortcuts for debugging, you have to change default settings.