I'm generally not a fan of them. As far as my opinion goes, some authors tend to lean on them, crutch-like, in order to provide an instant back-story for their characters. The transition between the (narrative) past and present, and also its placement in the story can be jarring, to say the least. Show, don't tell, is what I've always believed in.
Of course, that's not to say that they can't be used effectively. If it can be used in some sort of creative or innovative way, rather than just the cop-out method of 'Oh, but don't you remember when this happened? *cueflashback* way of writing, then it's gonna make your stories a whole lot better. I'm kinda thinking Kurosawa's Rashomon, or even Chris Nolan's Memento here. Yes, they're films, but they're pretty good examples nonetheless of good flashbacks.
Me? Still hate 'em, though.