Men in Black earns its PG-13 rating. It is very frightening (if your child scares easily, tell them no or be up at night for a week), and when it isn't frightening it's violent. When it's not violent the characters are cursing. This makes the movie innapropriate for anyone under 12, and if your 12 year old scares easily you might want to raise that age to 13. The movie follows a creative idea, but the story line is relatively weak. It is almost made up by good special effects and admirable acting by Tommy Lee Jones and Will Smith, but the blemish still shows.
Amazon.com called this an essential viedo, and I agree that you should definently see it once. Whether it's worth more than that, that's up to you.