Summary: The Dark Knight movie reviews are bursting out like a huge bat colony from a cave at nightfall.
Heath Ledger's performance is getting raves. Below are some selected quotes, with links to reviews I found interesting:

OfficialHeat.com

"I have NEVER seen a movie with this much hype not only live up to the hype, but
surpass any expectation you have due to it. ... It’s FLAWLESS. All the hype in
the world can’t properly prepare you for what they don’t tell or show you."

Passionforcinema.com

"I almost didn’t know how to absorb and analyze the movie at the same time. But
this is a high water mark in big budget cinema that almost seems untouchable."

AtomicPopcorn.com

"So first things first. Does Christopher Nolan’s “The Dark Knight” deserve
all the praise it’s been getting? The answer to that is a resounding
“YES!” With “The Dark Knight”, Nolan has not only crafted a film that
easily surpasses his original (and that’s saying something), but he’s also
created a truly memorable filmgoing experience that is without a doubt the best
movie of the year."

RollingStone.com

"I can only speak superlatives of Ledger, who is mad-crazy-blazing brilliant as the Joker."

Comcast.net

"So go, see "The Dark Knight" and revel in the fact that a "popcorn movie" just might be the best film of the year."

One word of caution. Several reviews said the movie is very DARK, and the Joker is no comic, he's a sadistic, evil killer. You may want to think twice before taking young kids.