After installing VMWare Fusion, I was really pleased at how easily it was all done. The system provides you with 3 options of installation - i.e windows from a CD, conversion from Boot Camp or import a copy of your PC. I installed Windows XP from an original CD and it was a complete breeze. You have a security option to decide whether you want to share all of your Mac data with windows or run an isolated process. I opted for the isolated option to protect my Mac data from the many PC viruses but then set up a sharing folder for the data I need to share with windows.
The settings are really useful and you can change how much memory or how many processor cores you want the Virtual Machine to use. You can also configure virtual drives if you want separate hard disk space and, although I couldn't assign my external USB partition as an actual drive, it worked fine as a virtual partition.
Now the not-so positives:
I am running this on a standard MacBook with snow leopard, a 2 GHZ dual core processor and 2 GB of memory. I assigned one core to the VM and left the standard allocation of 512MB of memory. Well, the first thing I noticed was that the virtual machine slowed to a crawl when I started running excel and Visual C++ at the same time. There was no real impact on my Mac apps so I changed the memory allocation to 1 GB each but that slowed it all down and it became such a pain I changed it back again. I need at least 4GB of memory if it is going to serve its purpose for me.
If you don't intend to use the VM and Mac Apps at the same time, and are happy to swap memory allocations, then it is perfect on any Mac.
If you want to run applications on both at the same time, ensure you have enough memory (2GB wont be enough).
UPDATE: I have now installed an extra 2GB of memory (now have 4GB in total) and it is superb !!. I have now upgraded my rating to 5 stars