Harley Quinn's Revenge is a downloadable content pack for Batman: Arkham City (which I've reviewed here). After the events of the main story have wrapped up, Harley Quinn is out for revenge (surprise!) and targeting Batman for his takedown of the Joker.
Warning: spoilers for the main story below.
Play as Robin
Hooray, I can play as Robin in not a challenge map! Actual story content as a character with cool new abilities! Too bad it's only an hour or two of content and you're still Batman half the time.
Harley's scheme does feel a little rough around the edges, but it's still too clever and contrived for it to feel like it comes from actual grief. Even when she actually manages to capture Batman she traps him in a bulletproof glass sphere and... well it seems like she wants to let him suffocate to death so that he "understands how Joker felt when he was left to die". That doesn't seem much like revenge, or like the Harley I know. I would've thought she'd want to torture him.
The weird thing is that it comes close to that point. Near the end it looks like she's managed to kill Robin - except he pops in at the last minute to save Batman from being stabbed.
Pretty much the entire function of the DLC is to tell you that yes, the Joker is really dead, and Batman is pretty messed up about it. I kind of feel that that message would have been better communicated with a short but powerful cutscene.
The area is very linear. Clear a room, go through the hall to the next room, clear it, hall, etc. While you can argue that it makes sense, it's still much more restrictive than other indoors missions in the game - the predator maps are small and present few options (especially as Robin), and there's no trying to figure out the route, you just find the door and go through it.
How Do I Play It?
Bought the DLC months ago, finally decided to play it. So I loaded up the game, aaand... nothing. Couldn't figure out how to get started. Checked the internet and it told me that I needed to redeem the key through Games for Windows Live. So I redeemed the key, and now it tells me I need to download 1.5GB of game data. Argh, GFWL. It's not hard, it's just that it doesn't tell you how to get started, and it's something I would have expected to get through Steam normally. Annoying.
This is a little weird. Normally you can hit Tab to access your map and a list of all your upgrades and gadgets and how they work. In the DLC it's just the map. If you're just coming back to Arkham City for the DLC after some time away, it's likely you've forgotten how to do some things (like disarm & destroy for me), but you can't check how from within the DLC. You'd have to go on the internet or load into the main game. It's worse for Robin, because it's entirely possible you've never played him before, and here you have these new gadgets (snap-flash, bullet shield) and the game won't tell you how to use them. Annoying.
Recommendation: don't bother.
It's short, uninspired, contrived, and adds nothing to the story. If you get it really cheap then sure, why not, but I absolutely don't recommend playing it immediately after you finish the main story. It reduces the impact of the ending somewhat.
It even retcons the pregnancy test easter egg from the main game.