The DLC actually picks up from the ending of Arkham City after the death of Joker (Though there's actually a comic series that ties the original game with this DLC). When Harley Quinn caused chaos and kidnapped several cops, Batman sets out to rescue them in Arkham City. A few days went by and there were no signs of Batman. So the journey starts with you being Robin searching for clues of Batman's whereabout.
Yes, you finally get to play Robin in a campaign mission. Cool! Robin has several different weapons at his disposal that does differentiate him from Batman's capabilities but the gameplay here is still the same so you don't have to worry about any getting use to (Unless you haven't been playing it for a while, like me). And at some point, you'll get to play as Batman again.
I agree with a review I read that this is how a DLC should be. The fact that this DLC adds additional storyline, new settings and a side mission makes it obvious that the guys at Rocksteady really cared about making a proper DLC. It also includes several combats with different degree of difficulty. However, don't expect anything new in terms of gameplay. This was probably one of the very few DLCs that I've ever completed cause it was still engaging to play. Worth getting it? You bet!