It's grim with a capital G. Also, a capital R, a capital I and a capital M. Also, all the letters are in bold. And underlined.
Why you shouldn't watch it: I'm pretty sure Christopher Nolan was molested by a man dressed as Batman in his impressionable youth. It's the only reasonable explanation for the sadistic mental and physical torture he puts his protagonist through. A far more accurate title of the movie would be 'The Dark Knight Falls Into A Deep Dark Pit Of Despair Where He Is Dragged Over The Hot Coals Of Hellish Agony And Unimaginable Pain From Which There Seems To Be No Escape...And Then Rises'. Granted, it makes the 'Rises' bit pretty impressive, but if your idea of a good time is 'The Avengers', stay the fuck away.
Why you should: On the other hand, remember when they tried to make Batman more lighthearted and breezy? We got Bat-nipples and Bat-built-in-skates and Bat-credit-cards and Bat-banter-with-Robin-about -who-gets-the-girl. So perhaps that's not the best way to go with a batman movie. Darkness and despair are integral to the Batman character, and trying to avoid them never works out well.
It's very much Christopher Nolan's Batman
Why You Shouldn't Watch It: Do you like your superheroes to be, you know, super? Do you wan't your villains larger than life? Do you love to see them revel in their power? Well, in the word's of Clint Eastwood in the worst Dirty Harry movie yet, you're shit outta luck. Christopher Nolan doesn't like his superheroes to leap, or even jetpack over tall buildings in a single bound, so Batman doesn't hit any harder or faster than a normal, albeit very skilled and powerful fighter. Nor does it matter that in the comics Bane is a hulking brute who can grow to the size of a largish SUV thanks to a magical toxin called Venom. In Nolanverse, he's the size of a man. A man who can crush skulls with his armpits, yes, but still, basically, a man. Everything's grounded, everything's real and nothing's larger than life.
Why You Should: You know what that also means? Everything's believable. Batman, with all his toys and training, is a man whose body is giving away. No comic book logic can save him here. Things have weight, especially the meaty hands of Bane as they pound poor Bruce Wayne into submission. We all know that in a superhero movie the good buy wins, the bad guy loses and everyone goes home happy. The fact that even this knowledge can't keep you off the edge of your seat is a testament to Nolan's ability as a filmmaker.
It's the sequel to possibly the greatest comic book movie of all time.
Why You Shouldn't Watch It: First thing's first. If you don't believe that The Dark Knight is among the greatest comic book movies of all time, you're wrong. There's no debate, no "it's my personal opinion", no "it doesn't work for me". I could throw statistics and data at you all day long about its 94% rating on Rotten Tomatoes, about ecstatic reviews from the best critics in the business, but the simple fact is, none of that matters. Your taste in superhero movies is judged by how much you love TDK, not the other way around. And therein lies the problem.
It's tough to follow the best. Unless you are some sort of superhuman non-expector, you are going to walk in with expectations, however, slight, of Nolan outdoing himself. And you will be wrong. There is no outdoing The Dark Knight. It's a good movie, great in parts, but it does not have a Heath Ledger as the joker. The story is clunky at times, and forced in others. And it goes to far with the whole "though shalt smile less than you did at your grandma's funeral" theme.
Why You Should: Because it's not just a sequel to the greatest comic book movie of all time, it's also the end of the saga. Nolan's Batman is not a collection of 3 stand-alone movies like the Spider-Man trilogy. The movies are tied together and without TDKR, TDK is incomplete. It does not take the tale of Bruce Wayne to its natural conclusion, and it does not provide the wonderful closure that this movie does. If you are emotionally invested in Batman, Bruce Wayne, or Nolan's Gotham at all, TDKR is a movie that you need to watch.