I saw this for the first time, recently. I'd never been interested, and might not have seen it if not for my 101 Things commitment. I was pleasantly surprised.
Dustin Hoffman does a great job as a career-oriented man who must learn to take care of his son and balance work with parenting. Justin Henry, who plays the son, is very believable.
The story doesn't become maudlin or overly comedic or preachy. It's just the story of an ordinary guy, clueless to what it takes to take care of a child, learning how to handle his new role while his son learns to deal with him as a parental figure.
It would have been enough to leave it at that, but the mother (Meryl Streep) comes back and tries to win custody of her son. At the time of its release, a father winning a custody fight was extremely unusual, and the courtroom settings are well handled.
There can be no good resolution to a situation where parents fight over their children, and the movie doesn't presume to create one. Instead, it ends as it went on, honestly.