Just this second finished it. It's disappointing in that it's incredibly short, the story doesn't go anywhere (it's not like Harley had any big plan or anything), it's completely linear with no room for exploration, and doesn't offer up anything new in terms of gameplay. It also has no boss fight at the end; between that and the story not resolving anything, it ends with a whimper.
However, I don't regret buying it. I enjoyed playing as Robin (I'm not interested in playing Challenge Rooms, so the fact that he's in those is irrelevant to me), and thanks to Arkham City's interpretation of Robin being awesome, it's the first time I've actually liked his presence in anything Batman-related. But more than that, I enjoyed having a reason to play Arkham City again.
All in all, I'd still say it was worth the 800 Microsoft points I paid for it. Barely. 400 would certainly have been a lot fairer, and I'd recommend it to anybody who owns Arkham City at that price. But as it is, I'd say only get it if you honestly don't mind combat-orientated gameplay for two hours, and nothing more. If you download it hoping for story, variety, or exploration, you'll be disappointed.
