Re-defined my experience with AC. Did not have that much fun in a driving game for a long while. I was not sure what to expect before downloading and using, but man it is good. Just give it a try, I'm sure you'll get hooked as well.
Very good Career mod, it allows you to make custom opponents and race with them. Also, I like the fact that the drivers you race against will change over time as well adding variety and making it feel more realistic.