The coolest thing about these games is they give you an inside look before the movie even comes out. A majority of the time the game is released before the movie is. This also as an effective promotional technique for the movie because:
1. Unlike many games, they actually run advertisements for them on television. (Attracts children)
2. The games are often cheaper than the standard video game. (Mom approved)
3. They are widly available. (Mom acessable)
4. Usually have a ESRB rating of 'E' or 'T' (Mom approved)
For some movies, these games can actually turn out to be a success. By being able to target younger audiences, the family is more likely to go see the movie. Also, the games themselves can be successful in design and gameplay. Lego Indiana Jones, most of the Star Wars games, the Harry Potter games and many more provide a hands on experience that you can't get by just watching the movie.
If you haven't played any of these types of games before, I'd suggest you check them out. Most of the games have the same story line of the movie with extra side quests, puzzles, or missions, which add to the experience. They're also great games to go out and rent and finish with a friend because most of them feature a co-op option.
One series that has never failed to please is the Lego game series. From Batman to Pirates of the Caribbean, they have plenty of humor and multiplayer action that is fun for anyone. They're easy games for anyone to pick up and are available for mulitple platforms.
Personally, my favorite were the Harry Potter games. I had them for the Xbox and the PC. It was a marvelous experience to be able to play as my favorite wizards and to explore his world beyond text on a page or images from a movie. What's your favorite?