As I type, The Dark Knight is the number 1 movie on IMDB's top 250 movies of all time. That will probably come down a bit. Um, maybe.
You've almost certainly heard about the plot, so I won't bother to summarize it.
All the actors here give outstanding performances. Heath Ledger has gotten the most attention for his interpretation of the Joker, but Aaron Eckhart also deserves some major respect for his Harvey Dent; without spoiling things too much, his character undergoes the biggest transformation in the movie. And Maggie Gyllenhaal brings an intelligence to the role of Rachel Dawes that was pretty much absent with Katie Holmes in the previous movie.
And Christian Bale really is the best Batman ever.
It's all very intense and bleak and (sorry, I know this is the word everyone uses) dark. (It's PG-13, but I wouldn't take a 13-year-old.) But it's not unremittingly dark; again without spoiling things, one major plot point near the end is resolved with optimistic inspiration. But the rest of the movie's gloomy enough so that's kind of a shock in itself.
It's also 152 minutes in length, with what seems like 20 minutes of previews on the front end, so the small-bladdered should plan accordingly.