Many things can go wrong with a movie adaptation of a cartoon TV show. First: the voices may be changed. Sometimes studios will change the lead voices and replace them with famous people's voices. This movie keeps Tom Kenny, Clancy Brown, Rodger Bumpass, Bill Fagerbakke and Mr. Lawrence of the original principal cast and only adds celebrity voices like Alec Baldwin, Scarlett Johansson and Jeffery Tambor as new characters created for the movie.
Second: The film might look nothing like the show. The character designs and background of Spongebob remains true to the TV show. This is a smart movie by the studio. Movies like The Rugrats Movie, The Wild Thornberries Movie and even Beavis and Butthead Do America have a distinct style and the studios polish it to the extreme. In the case of The Rugrats Movie, the following season of Rugrats looked just like the movie. Spongebob can be intertwined with episodes of the series and no one would be able to tell (except for the widescreen format).
Third: Because of the bigger budget that comes with the feature film, studios try to put in some computer animation for effect. Spongebob does this. To their credit, it doesn't look that bad. But they still shouldn't have done it.
Spongebob Squarepants: The Movie is the coming of age tale of a sponge of the movie title's moniker and his starfish friend Patrick, as they embark on a journey to find the crown of King Neptune. They have to overcome tremendous odds, free all the fish from the clutches of Plankton, and defeat the bounty hunter, Dennis (Baldwin). Oh, and David Hasselhoff makes a cameo and is a major player in the third act. (This is, by far, his best film to date)
The story is self contained and the characters are introduced appropriately at the start of the film. You do not have to be a follower of the show to enjoy this movie. But you do have to be a fan of absurd humour, like Ren and Stimpy. There is also enough humour that kids wouldn't understand to keep all adults entertained. The laughs are huge and the animation beautiful. You even get to see Spongebob drunk and hungover and Hasselhoff's hydraulic pecks!
You may not believe me, but this movie is worth watching whether you are a Spongebob fan or not. Honest.