I really enjoyed Slumdog Millionaire. I went in only knowing it was a "rags to riches" story revolving around the India version of the "Who Wants to be a Millionaire" TV show, which sounded a little trite. The story ended up being a very powerful story, and in the early part highlighted the amazing squalor the poor in India lived in. The violence and cruelty were also pretty extreme. The last 25% of the movie turned into a little bit of a predictable soap opera on the order that you might see in a TV drama, but the first 75% definitely made up for that with its intensity and raw portrayal.
"The Dark Knight" is a great movie, perhaps the best of this summer (or at least neck and neck with "Iron Man"). Heath Ledger's Joker is the best thing about the movie, with the plot and action sequences all being quite good as well.
My only complaints are the movie's length and overly convoluted plot by the end, as well as an ending that is a little flat (it just sort of peters out) after such a good beginning. I think if they had cut out the entire Two Face sequence, and maybe 30 minutes of movie, it would have been a lot better. They were just trying to get too much in by the end, and seemed to lose their way in the plot complications.
Iron Man is a pretty impressive superhero movie. As a comix character, he's pretty boring, but RDJ pulls off a good Tony Stark and makes for an interesting story.