Very good. Deliberately epic in scope; flawed in execution. Let's start with an audience note, then get the bad stuff out of the way before being not queasy to praise a little.
I get the impression that it's a fandom faux pas to make a sweeping statement that a particular thing is "not for kids" or "suitable" for same. Every child is a unique snowflake, and the children of nerds, even . . . flakier. Fnord knows mine are. So, I'll just say: The movie includes children being menaced close up and at length, ordinary people being deliberately tempted toward monstrous behavior, lingering scenes of bald cruelty and the drawn-out death of a sympathetic character. As a minor matter, Harvey Dent's late-movie makeup job is pretty effective. I'm sure my eight-year-old daughter would find the movie too upsetting to enjoy, and I suspect that is true of my twelve-year-old son too, if not in the same degree. Your mileage MV.
Now the bad stuff. Some of it is pretty major:
(1) The script is not so good. I'm specifically talking about the words the screenplay puts in people's mouths. There are an awful lot of Moments of Shit. Part of this seems to betray an anxiety I thought superhero movies had outgrown: the compulsion to make you understand that this story isn't just about the fights oh no. It's the cinematic equivalent of the "Bap! Ziff! Pow! Comics Aren't Just for Kids Anymore" headlines that appeared at the top of every "Gosh, there are a lot of 'comic books' for adults these days" article a few years ago. The Nolans want to make sure you understand that this movie has themes, and what better way to get that across than to come right out and tell you?
Alas, some of the dialog doesn't play at all, even in the mouths of what is a pretty good cast. And the Nolans don't seem to be able to write chords, only single notes. They haven't mastered having characters talk about one thing in terms of another thing; overtones and even dramatic irony seem outside their purview.
(2) A problem from the first Nolan/Bale Batman recurs. To the creative team, it only makes sense that Bruce Wayne would disguise his voice when under the cowl so nobody can recognize him from listening to him. And that's all it does, is make sense. It makes the movie worse as a drama, because the particular guttural mutter Christian Bale uses when in costume has zero expressive range. Every now and then he gets to shout (it sounds like the vocalist howling over a death-metal coda), and that doesn't sound too compelling either.
(3) This edition of the Batman costume struck me wrong. The join among cowl, cape and body suit looks kludgy, almost slapped together. This doesn't even had dramatic justification, since Bruce Wayne's resources (human and financial) could surely do better. And the cowl itself is distractingly pear-shaped, with a big bowl for the cheeks tapering to a narrow bell for the crown of Bale's skull. The ears point inward and the bridge across the nose and cheekbones is distractingly indistinct.
The above strike me as big honking deals. Nevertheless, this is a very good movie. It's not the best superhero movie ever: tonight I don't think I'd put it in my top five. Ask me tomorrow. But it's very ambitious - it really wants to be an epic - and it achieves a lot of what it wants to achieve.
(1) The story, as opposed to the script, is very good. The general course of the action weaves together several plot strands in ways that make sense as action and fulfill the themes of the movie. The story is about what kind of coward you are or aren't, with cowardice being the willingness to have others pay the price for what you want most. Courage, in Dark Knight, consists in letting go of your life or dignity or reputation or ambitions for the sake of others. The Joker would like to show that courage doesn't exist. The beats of the plot vivify the struggle beautifully, so beautifully, you wish the screenwriters would stop having people tell us what it's doing.
(2) Heath Ledger really is terrific. The creative team's very smart plan is to simply go around Jack Nicholson's take on the character under Tim Burton's direction. This would only make sense anyway - you're not going to be more Jack Nicholson than Nicholson was - but especially so since I don't think Nicholson's performance has worn well. By which I mean, in retrospect it kind of annoys me. A lot of scene-chewing passed off as an interpretation. This movie's Joker underplays himself, and Ledger underplays the Joker. He shuffles; he mumbles; he pretends to play low-status. He's also cunning, usually a step ahead of his opponents, a true wild card. Aspects of his demeanor are almost professorial - the rumpled sort of serial-killing professor.
(3) Most of the rest of the cast is serviceable or better.