I liked Crash Course, but it's buggy and short. But if you liked replaying the other maps etc, you'll replay this one a decent few times, and if going for the achieves, 10-20 replays etc, which imo = 10-20 hours, which is worth 6 or 10 bucks or however many points it was.
I haven't played Borderlands DLC yet, the Zombie Island looks really fun, but sort of short for $10, the underdome seems like there would be a decent bit more bang for your buck though.
Mass Effect has 2 DLC's and if I remember right, both under 2 hours from what I heard.