Engaging story, great action, twists, and surprises. Heath Ledger definitely adds a more disturbed character to the Joker role. The discussions of 'Oscar Possibility' for Ledger are not patronizing due to his death; he's earned a nomination, IMHO.
PG-13? I dn't have pre-teens yet, but I think younger kids would be quite scared by this movie. I saw many parents taking 7-to-10-year-olds into the theatre, but I for one wouldn't take my kids yet. No preaching here - do what you want - but just wanted to pass along my $0.02.