A Mac Mini became available and I bought it. I wanted it for my Objective-C iPhone development. It came with a bonus - the discs for Mac OS-X Server. So before I set it up with Snow Leopard and Xcode, I installed the server and did some exploring.
First of all, it's only Leopard Server, a 10 user version. This was quite expensive when it came out at $1,000. I investigated an upgrade to Snow Leopard but there is no upgrade path. The Snow Leopard server is available for $500, for unlimited users. No upgrades, just pay full price. I won't be doing that.
I played with it a little and it's interesting. Very pretty, everything done nicely the Apple way. If I didn't already do all this with Linux, I'd be very tempted to go further down the Apple path. But I already do everything with Linux, and I need the Mac Mini for development, so I installed Snow Leopard on top (over some strong protests from OS-X Server) and set it up and got back to my iPhone programming.