While the plot of this movie is relatively interesting, this movie is really about two things:
First, the lead actors get to do the characters they do best as fully as they know how. Cage is his typical, neurotic, repressed, and emotionally conflicted self. Travolta hams it up in the outlandish, larger-than-life way that he often does. Despite what could be seen as overacting by both the leads, the almost absurd disparity between them is one of central things that makes this movie work.
Second, this movie has the most beautiful action sequences of any action movie I have ever seen (excluding epic-type movies, i.e. LOTR, Star Wars, Braveheart, etc.). This is a film that knows it's an action movie and revels in it. The cinematography in the fight scenes simultaneously displays the nature of the characters and the intensity of the battle between them. There are so many moments when you will just delight at the visual art that is every action sequence in this film. Needless to say, this movie has all the gunfights, fistfights, chases, and large explosions that one would expect in a serious action film.
If you like action movies, this is the first one I would recommend.