For anyone that's used this before, how well does it run? I always wondered if you ran something like this all the programs would run a lot slower than they would on an actual PC machine.
Not really. VMs do have a performance hit, which will be more noticeable when running applications that do low level things (I.E. games hitting OpenGL drivers, or audio apps expecting ASIO drivers), but for general desktop work, the experience is plenty good enough.
You do need a lot of RAM though. OSX wants ~2GB just running idle, and Win7 is going to want 2GB also, so 4GB of system RAM ion your Mac is a minimum, but more is desirable.