As with all things in writing, some prologues are good, and some are not. Some prologues are very useful and interesting; others detract from the story. For every prologue you read in a published book, ten more were probably discarded along the publication route.
But then, the same can be said for works in general, epilogues, regular chapters, etc. I don't see what makes prologues so different, other than this hyped up irrational hatred that some people seem to have for them. Prologues are just another story element to me. Sometimes, I find them appropriate for my works. Other times I don't.
For example, I have a prologue for one of my dystopian stories that depicts an event that happens several months before the real plot begins to unfold. Not only does it introduce a few key concepts that are vital to understanding the story as a whole, but it also (unbeknownst to the reader the first time through) contains 1) the event that triggers the entire plot and 2) foreshadows the entire story to come via a careful layout of subtle clues, both of which are very important to know when the big reveal and climax roll around.
Without the prologue, I'd have to stick a lengthy flashback in there somewhere or have my character "remember" and describe the event some point, which I hate doing unless it's absolutely necessary. In this case, it's not. In my opinion, the prologue flows very well into the whole story and gives a decent and well-paced introduction to the setting and protagonist without info-dumping. Not to mention stuff actually happens in it. (I, too, have a strong dislike of the prologue where nothing really "happens.")
On the other hand, I would never begin my 1920's historical fantasy with a prologue. It doesn't need one. I pick up...well, not exactly in medias res, but after the real "beginning" has happened. In this story, I do flash back to what happened in the beginning in bits and pieces, because it works well with the way this particular story is told. In the former story, such a technique would really disrupt the flow.
Basically, whether or not to include a prologue depends on the particular story, in terms of both plot and writing style.