Finished this three times now looking for that last Harley Balloon, I'm convinced it's glitched out on me because I ended up following a guide and it didn't point me to any new ones.
My first run through took me about three hours, but I'm a slow player. Then subsequent runthroughs, when you know what you're doing, can take less than an hour. I enjoyed it, but when those credits rolled I did wish there was more of it. It's short and simple but enjoyable.
I really enjoyed playing as Robin, he has subtle differences to Batman which were refreshing and yet familiar. I missed certain moves, such as the ability to grab enemies from below perched positions, but generally enjoyed beating people with a big stick and carrying around a shield to protect myself from gunfire.