I've been in discussion groups, discussed this topic several times. Some people love prologues, some people hate them. Myself, I love them.
When I write a prologue to a story, I'm basically trying to hook the reader right from the very start. I want to hook him and reel him in so he'll go on to read the first chapter. The prologue should set the tone to the book. It should give the reader an idea of what's to come later on in the story. This is very important. This is how you hook your reader.
You can use a prologue or not, but I say use it. It's the best way I've found to hook your reader and reel him in.