I think most of the other posts are saying, more or less, what I think. It really depends on the content and writing in the prologue and on the author's reason for including it.
Prologues that don't always work for me include ones that contain a bunch of dull, telly world building or background that doesn't read like an actual scene (these were once pretty common in SF and F, and I tended to skim, or skip, them unless they were really interesting). Also, I tend to dislike ones that get me rooting for a character who won't be in the main story at all, because they die at the end of the prologue or because they did something that's really far removed from the story's main events. They tend to work better for me when they show the protagonist, or someone else who will be an important part of the story, doing something that takes place some time before the main story starts (but not lifetimes before, generally) that is the catalyst for the main story arc that commences months or years later.
Mainly, they need to be so interesting that they get me reading.
An editor (for a big five subsidiary) who spoke at a writer's workshop I attended said she tends to dislike prologues personally, but added that since about 1/3 of recent, trade-published novels have prologues, maybe we should take strong statements about how much agents and editors (and readers too) hate prologues with a grain of salt.