There are clearly two Robin Williams.  One is the hyperactive, slapstick, mile-a-minute guy who works perfectly as the voice of Aladdin's Genie but who also takes roles in crap like RV.  Then, there's the serious actor who brings great comic timing but also some gravity to films like Good Morning Vietnam, Good Will Hunting, and The Fisher King.  I can take the former in the right context, but only in small doses.  The latter usually works better, but every once in a while he picks a role in a film like One Hour Photo where he goes a little too far from his comfort zone and he just seems out of place.  I'm happy to report that Williams' dumpy high school teacher in World's Greatest Dad is exactly the kind of character that he should be playing.  He's a likable, middle-aged high school poetry teacher trying to raise the world's most reprehensible teenaged boy.  Williams is funny and charming but it's easy to understand how he gets walked all over and how he is the typical nice guy finishing last.

As I mentioned, Williams' son in the movie is a real piece of work.  He's completely irredeemable and at no point does he even begin to earn any sympathy.  Every scene with that kid is uncomfortably true and I found that the only way to enjoy those scenes was to laugh at how absurdly unlikable he was.  This is the kind of humor that is probably not going to work for a lot of people--it's honest but dark consistently offensive.  The film's first act paints such a vivid picture of the kid that it doesn't even seem like hyperbole--there are real kids like this.  Hopefully they outgrow this 'I hate everything' phase, or they only use that persona online.  The message boards of the world are full of these trolls, but rarely do we see them acting out like that in real life--here, we do.

After the film's turning point (which the trailer doesn't spoil, so I won't either,) the movie plays more like a satirical farce for a while.  It's a strange shift because the tone of the film doesn't really change, but its veracity does.  Background characters become less believable and situations start to strain the boundaries of credibility.  Through this part of the movie, it started to feel like the film was actively trying to send up typical teen/high school movies and that didn't always work for me.  Still, the film does an incredible job of allowing Williams' character to dig himself deeper and deeper into a mess that propels the film through its final act.

World's Greatest Dad finishes well, which saves any dragging points during the film's middle third.  While it skates between satire and just straight, dark comedy, it ultimately winds up telling a story that needs to be told.  It's part cautionary tale about raising children and part hopeful reflection on the ability that we all have to change our direction in life.  Sometimes the catalyst for those kinds of changes isn't pleasant, but the movie is all about demonstrating how the choices that we make at those turning points are crucial.  If a movie like Parenthood is too sentimental a reflection on parenting for you, then World's Greatest Dad might be just what you are looking for.