Heat is a slow movie. In a sense, this is a good. Rather than cram a sequence of action-packed scenes into 90 minutes, you have twice as much time to slowly digest each one of them. On the other hand, if you're used to 90 minute bang-ups, this one might appear to take forever. Heat manages to make me feel both: throughout the movie, I never felt like gripping my seat handle, but I did enjoy laying back and taking what little of the story there was in slowly.
Heat's plot is an Hollywood cliche: Al Pacino plays Vincent Hanna, a cop, who's the good guy. Robert De Niro plays Neil McCauley, the head of a criminal gang which includes Chris Shiherlis (Val Kilmer) and Michael Cheritto (Tom Sizemore). They are the bad guys (duh). Hanna must stop this criminal gang just as they are about to pull off their last big job. The movie has a moralistic ending (yes, I just gave the plot away) which makes a feeble attempt at surrealism.
However, one shouldn't see Heat for the story, or even for the action. Heat is mostly about character development. We rarely see a gangster's personal life scrutinised in detail in Hollywood movies and Heat is a welcome change in that direction. Heat points out (perhaps not too surprisingly) that most cops and robbers lead pretty normal lives with normal problems, i.e., when they're not toting automatics, and robbing banks.
However, Heat turns out to be somewhat of a self-parody. It's hard to empathise with the robbers who kill people on whim. It's also hard to empathise with the cops who don't seem to really care about people but are simply interested in winning a gun fight at all costs. While the acting (and cast selection) is brilliant, Heat might've been better off not being a cops and robbers movie.