Recommending getting this DLC or not becomes a moot point when it is not bundled in with the GOTY Edition of the game, and is showing, as I write this, at £7 for the whole bloody lot. Since my recommendation is to definitely get the main game, you'll end up getting this anyway.
Now as for my experience, well, I bought the full game early on, so I got suckered in to getting this as DLC for a fiver (I think it was on offer at the time, fortunately), and what do you get for a fiver? Not a lot, really. The DLC mission took me all of 2 hours to complete (and that's including a break or two and an extended "I'll just leave Harley hanging there for a few minutes because it entertains me" moment of madness), and barring missing a few balloons to pop here and there, there isn't really any reason for me to go back and play it again, and I'll emphasize this part here, because when you complete it there's no going back to get the remaining missing items like the main game, you have to start the campaign from the beginning.
The brevity, in a way, is welcome to a certain degree thanks to Robin's cumbersome controls. Yes, you play as Robin instead of Batman or Catwoman or Bob, and while he has a couple of unique abilitiies, his overall combat controlling is not as smooth as the remaining characters. Combos break all the time (which for someone like me is normally unsurprising, but I'm not THAT bad, for crying out loud... this made me look like a complete novice at the game!), the snap flash bang whallop, or whatever it is called, did not get used once, which is lucky 'cause I couldn't figure out how to use the damned thing anyway, and the hollow "clunk" of the stick just isn't as satisfying as the meaty "thud" of fist against thug.
The voice acting isn't up to scratch compared to the main campaign either. One particular note-worthy line from Batman: "The code, give it me." Really? REALLY?
Were it still a separate item, would I get it having seen it? No. Should you play it if you get it as part of the GOTY? Yes, it's worth it for that. Don't go in expecting any significant plot additions, or any hints at a sequel, though.