I find it very amusing to read some of the comments regarding the length of the film and how the relationships were just 'fillers' and not needed. I suspect you are from the 'bored in 15 seconds' brigade that are unable to concentrate for any period of time. The relationships are what the movie IS about! They aren't filers, they are the backbone.
Heat is the greatest drama ever made, end of story. Every time I watch the movie I pick up on something new. The husband/wife, boyfriend/girlfriend cop/cop, robber/robber relationships are the crux of the movie. The scene between Pacino & De Niro is the extension of this - Mann has brilliantly written a script that that is the best cop/robber movie ever, and centred it on the relationships of the various people which then (almost) culminates in the meeting of these two, and their relationship. Somebody commented on why didn't De Niro drive off with Brenneman. The answer is given in the movie, and forms what the movie is about.