I set up a Windows domain running 2008 R2 server under Fusion on my Mac Mini.
I’m very impressed. Those Minis have yoyoed rather more than I’d like (OS X Server), yet the Windows servers have been rock solid. They just work. I’ve not had to spend much time maintaining them; almost all my work has been in extending my virtual windows network. I have made some configuration errors that I’ve had to fix (e.g. having a DHCP server with a short lease without a failover is asking for trouble), but these problems have been with my lack of understandings, not a problem with it actually working. Indeed, these type of problems are precisely why I’ve set up this network: I want to learn, and I’m doing so.