Quite possibly the best multiplayer racing game available on the Xbox360 today.
Criterion has implemented a simple yet effective 'revenge' system which tracks all the people you've crossed paths with in previous races. So if you've pummled some poor guy into the wall a number of times you'll be pride of place at the top of his revenge hit list the next time he joins a race that you're in.
At the start of each race you can instantly see who you've taken down before (denoted by a green exclamation mark) and those who are looking to settle the score with you (denoted by a green exclamation mark). Just to hammer the point home there's a quick 'fly by' of the grid highlighting the people looking to settle a score with you and those you should be looking to ram into the nearest bus.
I would highly reccomend that newbies to the Burnout series finish the single player game before venturing online (unless you've got a lot of patience). Almost all of the player created sessions are played with the fastest available cars, so the sheer pace of the races can be punishing to a new comer.
All in all, even if you have a passing interest in racing games - give Burnout Revenge a try. Singleplayer is brilliant - but it's the multiplayer mode that really shines.