The best revenge is living well, that or building a deathtrap dungeon

The two facts I could remember about Batman: Arkham City - Harley Quinn's Revenge were that it was (1) good, and (2) short.

Like, I didn't distinctly remember that Robin is in it. That's how un-memorable this DLC is overall.

It is still good, though. Harley Quinn's (brief) Revenge tells a fun little story that includes some combat, some stealth, and even a puzzle or two. It doesn't bring much new to the table - other than Robin's shield, which ... doesn't work very well - but it's also short enough to not wear out its welcome.

Better than: Fallout 4: Vault-Tec Workshop
Not as good as: Batman: Arkham Origins - Cold, Cold Heart
Pretty underwhelming use of a new playable character: but it does serve as good prototyping for the larger cast in Arkham Knight.