I have heard from a number of editors and agents that they don't like prologues. Often, the prologue is a crutch to prop up a novel that's beginning in the wrong place (or, sometimes, a novel that is about the wrong people). The prologue often features an event much more interesting which is dragged there from the past or from the future to start things off with a bang. Many beginning writers do this because they feel they need to set things up at the start of a book: to show the characters doing ordinary things, to give a tour of their world or a history of something that will be needed later. So there are a few chapters of not-that-interesting stuff. But the author knows he or she needs a stronger opening, hence the prologue, because it is easier to write a new and exciting scene than it is to sacrifice thousands of words that are not serving the purpose of the book.
That said, a brief prologue which provides useful and tantalizing information the reader will later feel pleased to have can enhance the work. My editor actually had me add a prologue to my second novel because she felt the reader might struggle with sympathizing with the protagonist. The prologue established the truth about some of his early relationships and showed a moment that strongly shaped who he is in the book. I think the prologue worked in this case because all of the stuff that happened and the characters involved are very important to the narrative that occurs, and this scene allowed me to introduce my character from a very different view.