There's a natural order to sequels. Not all films follow this route but this is how I'd say most film series end up playing out.

Film #1: The film is a smash success and leave people wanting more. And "people" could mean the audience or a cult following or a studio head who is in desperate need of a hit and thinks that remaking a film is a sure thing.

Film #2: A carbon copy of the first. The latest example is probably "National Treasure: Book of Secrets" which was basically the exact same movie as National Treasure. Sometimes, in cases like X2 or The Godfather, the sequel could actually build upon the first movie but more often than not, the movie is a watered down version of the first film.

Film #3: This film is usually terrible. Whether it's because the story line was spead too thin or too many people are now involved with the production or because the stars refused to do it or because everyone was just in it for the paycheck, third films almost always struggle. Usually, this is where most film series end.

Note: In some cases, films 2 and 3 are reversed, the second one sucks so they make a third one, which to ensure quality, is basically a rehash of the first movie (see: Ocean's 11)

But some are lucky to get to a fourth movie. And when you get to a fourth movie, all hell breaks loose. Fourth movies are usually the height of silliness. Often it can work (Live Free or Die Hard, Rocky IV) but other times it fails miserably (Superman IV, Alien: Resurrection). For Fast and Furious, I have a feeling that its going to work out and give us a movie that would be the thrill ride of the summer if you weren't laughing so hard at it.