Everything you've heard about Ledger's performance is true - I think he should at the very least be nominated for an Oscar. He absolutely disappeared into the role and while he's onscreen I challenge you to take your eyes off him. He's mesmerizing and scary as hell. The subtleties he brings to the character combined with the way he was written pack a gut-shot one-two punch. You're going to love his magic trick...
At the same time the film is also (in my opinion) the closest to the spirit of the Batman of the standard comic books in that our hero is more of a detective in this film than he's been in any to date. The Dark Knight felt much more to me like a crime drama along the lines of The Departed or Heat than it did a comic book movie - and that's what caused me to question it upon first viewing. But when I watched it a second time I saw how this was really the most faithful big screen incarnation of The Dark Knight ever done.
For the most part, the comic book world of Batman is not fantastic - it's more like The Punisher's world - taking place on the mean streets of a corrupt city, dealing with mortal (if extremely dangerous) enemies, and that's what we're given in The Dark Knight.
Another shining star in this film is Aaron Eckhart as Harvey Dent / Two Face. Eckhart does a great job and it was essential to the film that you really believe his inherent goodness and role as "white knight" of Gotham. When he finally falls it's believable and not cliched. As a matter of fact, anywhere the movie could have veered into a cliche - director Chris Nolan takes it in the opposite direction. Really Dent's fall is the primary story arc in the film.
And then we have the IMAX factor - If you are fortunate enough to live within driving distance of an IMAX theater, I recommend you watch the movie in that format. If you can't get in on opening weekend, go see it on a regular screen and go see it in IMAX the following weekend to experience the difference.
The scenes filmed in that giant format just swallowed you up. Many of the action sequences were filmed in IMAX and the effect really fills your senses. It's as close as you can get to feeling like you're in the movie without 3D. The opening sequence with the bad guys swinging out from a skyscraper and the scene were Batman drops from the top of a Hong Kong skyscraper will almost give you vertigo. And you're going to love the scene where you first see the Batpod. :-)
Should you bring the kids? I wouldn't (and didn't). This has scenes that are right up there with PG-13 horror films in not only imagery but situations as well. This Joker is no Jack Nicholson with a deadly hand buzzer - he's brutal when beating on his adversaries and his favorite weapon is a knife. And there are a couple of scenes that are pretty much PG-13 versions of what might come out of a Saw movie.
So after all that, why not a 5 star rating? There were just a couple of things that didn't quite make sense to me that I won't spoil here, and the film did feel like it ran too long at 2 1/2 hours. There did reach a point where I was ready for it to be over, yet it went on for another 20+ minutes.
In conclusion, The Dark Knight is the most "grown up" comic book superhero movie made to date. While I wouldn't dream of Iron Man being nominated for an Oscar despite how great and entertaining I found it, The Dark Knight is at that level, and that's quite an accomplishment for this type of film.