I work on a small one (15 nodes) all the time, but as a user/developer not an admin. I've built one or two before as well.
Once it's up & running well it tends to be not to bad, particularly if you have a good system in place to push out updates and such. More hardware maintenance of course - even on our small cluster we have some type of hardware failure maybe every other month. Mostly hard drives, the occasional power supply, or CPU.
Depends on what the users need as well - one of the guys on our uses some software that has to be licensed on each CPU, and that was a PITA to set up. Our bigger corporate cluster (~300 nodes) runs Sun grid engine and I've heard that is a PITA to install and maintain. They also have a much larger and more diverse user community as well which I'm sure comes with its own headaches.