I've never owned one, but I certainly wouldn't consider them useless. They're probably sufficient for calling bingo numbers down at the local nursing home, or making announcements at a company meeting. For live music though, I wouldn't expect a lot out of them.
Maybe an acoustic trio playing quietly in the corner of a restaurant or something like that. A band in a bar setting? Wayyyyy under powered.
I used one (house P.A.) at a Happy Hour/ Restaurant gig two weeks ago. Honestly I was a little skeptical before the gig (duo - guitar/bass, 2 vocals) and almost brought some extra gear - but decided to just go with it for the convenience and not worry. I'm glad I did 'cause it worked out fine. I think it was the "500" 4 mic inputs, 2- 8" drivers with tweeters on tri-pod stands. I was pleasantly surprised - the sound was actually pretty good (a little boxy sounding) and volume was not an issue at all. All this being said, I probably wouldn't go out and buy one (if one was offered as a gift I'd take it, or at a ridiculous price I might consider buying).