I haven't used them, but I haven't heard anything bad about them yet. As long as it is a general purpose server, you shouldn't have a problem, but if you want it to be a domain controller or something, you may have problems.

Just an FYI.... it isn't darwinBSD. Darwin is the name of the open source part of the OS. The actual OS is based on NeXT (which Steve Jobs invented). NeXT was a form of BSD using the Mach Kernal. That is what is open source in OSX. In other words, OSX is based on BSD with the Mach Kernal, and released the source for the kernal as darwin....