Fast User Switching hangs my server when it is running headless (which is all of the time). For some reason it needs a monitor connected for it to work.
What I view as unfortunate is that I have to keep it logged in in order for some services to keep working properly (scheduled backups with SuperDuper, DynamicDNS, I suspect Plex, and iTunes if I were to use it's server). So it isn't possible to use the server as a general user login on another account.
Of course some people will point out that a server shouldn't be used for user logins anyway for security and stability reasons.
You can always have multiple shell logins, using the ssh command.