I've used it as a mail server, a web server, and a database (postgres) server. It's also my main desktop OS. Did/does fine, but I never really stressed it. I would certainly welcome a more capable filesystem option, as well as something like logical volumes, but I can't say that ufs has ever failed me.

You'll definitely want to have it on a UPS to avoid some potentially long and sometimes manual intervention on fscks after a power failure. And of course, backups for anything important.