This is a good movie. Sure, it won't win any awards, but it's still a lot funnier than many more successful films out now. Some people seem to have a problem with mixing humor and Alzheimers. My own grandfather had this horrid disease and eventually passed away a few years back. Does this make me feel that this movie is less funny? Not for a moment. The reason is that nothing bad ever happens to Dom Ameche's character. He sets up Tom Selleck time and time again for painful slapstick gags, but he always comes out squeaky clean.
I found the writing witty with some good one-liners and great character interaction. On a believeability scale, it rates pretty low, but it is plausable. I saw no plot holes that others have mentioned, but it's easy to see plot holes when you already know the concept of the story, unlike the characters who are in the dark and haven't seen a "trailer" of their lives. The cast is also great, btw.
All in all, this movie is a great distraction, but if you haven't smile or laughed in awhile and you think South Park is immature, you probably won't like this film. If you do see it, just remind yourself that they are not making fun of Alzheimers, they are poking fun at the misadventures one man has trying to take care of his aging parents while trying to retain his own sanity.