OS X Server is great for its ease of use and being able to get it going just using the GUI. Some things you do have to mess around command line though. For mission critical uses it may fall a bit short though.
It's fine in smaller environments... in my testing I have found that it doesn't deal with high loads very well at all.
I took a G5 Xserve and ran OS X and RedHat Linux on it... a simple test with Apache was the target.
In doing stress testing... the Mach kernel after a mere 16 simultaneous connections would start to fall behind the Linux installation, by the time we were at 32 simultaneous connections, the Mach kernel was destroyed.
I used NetBSD and the difference was very similar... there is something not right about the Mach kernel under high server based loads... That's just my take on it.
On a desktop, these issues don't show up and it's not even remotely close. On a Desktop OS X is a clear winner.