The first UPS I ever bought was in 2001 when I was hosting my Web sites from home. That way if the power went out, no problem, sites stayed online. Indeed the following spring, I had a blackout that lasted an hour and the UPS held up no problem.
But since then I've had three of them, and at least two I can tell you are crap. One I've thrown away, and the other one, about nine months old, is supposed to power my Mac Pro. Well, the power just went out, and it lasted all of five fucking minutes. That's not the 30 minutes it promised. I'm particularly annoyed because a backup to S3, in the amount of 13 gigs, got interrupted, and since I don't have the super fantastic version of JungleDisk, that means it failed.
The other UPS that I have no opinion on powers my router and cable modem, and I have no idea how long it will last. It's almost three years old.