I wrote this as a reply to the review that GameTrailers posted for Batman Arkham City's post-game story dlc, Harley's Revenge. No worries; no spoilers.
"In defense of Harley's Revenge, I'd like to present a very fitting
metaphor. Arkham City is like a great comic book story arc. It tells all
sorts of great stories while also having a great overall narrative that
wraps everything up nicely. However, as is the case with Harley's
Revenge, most comic book franchises, even if they had an awesome story
arc, must continue. Harley's Revenge is like reading a couple of issues
after that amazing story arc. It's not terrible; in fact, it's still
pretty enjoyable. However, after such a grand and conclusive story, it
is hard to get into a new arc. Here's to waiting for the next grand
story arc, even if it means slogging through a few lackluster issues
That's how I honestly feel about Harley's Revenge. It simply doesn't quite fit into the old story arc. It compliments it, it continues it, but for all intents and purposes, it is a part of a new story arc rather than being a part of the old, concluded arc.
I will say that it is more substantial than a couple of the side-missions within Arkham City. The Mad Hatter's arc, in particular, is probably the poorest "stand alone" story within Arkham City; it's one mission, ends quickly, and isn't really resolved in any sort of concrete way. To use a comic book analogy again; the Mad Hatter section of Arkham City isn't even an issue's worth of content. Instead, it is a side story within a 25 page comic. It's moderately entertaining, especially for fans of the character, but it doesn't have any real place in the overall story arc at all.
Another mission that falls a little flat is the Bane mission. It's essentially an extension of the Riddler challenges; simply track down and "collect" the objectives, although in Bane's mission, "collect" means "blow up". It's much less complicated than the Riddler challenges, and even though the ultimate outcome is worth seeing, the mission structure itself is incredibly dull, even compared to the Riddler challenges. There was some room for additional story content that could have involved the leftover Titan formula, but it doesn't come together.
So both from a gameplay and story perspective, Harley's Revenge is better than some of the content that's already in the core Arkham City experience. However, it's still pretty average and ultimately doesn't add anything to the narrative of the overall experience. At the very least, I doubt anyone would argue that it is a "must-see must-play" piece of content. Whatever happens in the 3rd Arkhamverse game, I very much doubt Harley's Revenge will factor into the story in any way other than a passing reference.
-It's still Arkham City (so the gameplay is tight, responsive, etc.)
-The voice acting is still top-notch
-Playing as Robin within the actual 'Arkhamverse' is exciting
-The actual storytelling method is cool (switching between Robin [in the present] and Batman [in the past] at key narrative points to finally have the two stories converge is interesting)
-Harley's goons have some cool designs.
-Doesn't add anything substantial to the [fantastic] narrative of Arkham City.
-Essentially retcons an interesting story point hidden in the original post-story *SPOILERS* that Harley was pregnant (and now she isn't [and probably never was] apparently).
-Not being able to play with Robin outside of the refinery is a little disappointing.
-Aside from a couple unique puzzles for Robin [that both involve using his bullet shield], there isn't anything new to play around with.
-It's all over too quickly (roughly 2 hours, give or take)
So if you have already bought Arkham City, I wouldn't shell out $10 for the content by itself. For $5, it'd be easier to recommend. If you purchased the GOTY version of Arkham City, then by all means, play through Harley's Revenge. It isn't terrible, and in many ways it's above average, but after the stellar ending that capped the original Arkham City's story, it's basically impossible to live up to the original ending. Unfortunate for Harley's Revenge.
*SPOILERS* I'm hoping that Scarecrow, assuming he is the main antagonist of the 3rd game (given all of the hints dropped around Arkham City), plays on Batman's new failure/fear; the loss of the Joker. While the loss of his parents was the primary focus of Arkham Asylum's Scarecrow sections, I wonder how the loss of the Joker will affect Batman when exposed to new Scarecrow toxin.*SPOILERS*