If you are willing to turn off your intelligence, let the momentum carry you over the glaring plot holes, and ignore the derivative nature of this movie (Speed in an ice cream truck) you may come out of the theatre happy. Otherwise, prepare to be disappointed.
The surface problem with this movie is its inability to decide whether it is an action film or a comedy. Some films - notably the first Lethal Weapon - manage to straddle the divide to good effect, but not this movie. The violence is gratuitous and far too graphic for comedy (both the opening and the climax are unsuitable for the weak of stomach), and the comedy is too broad for an action movie, as well as being forced.
However, what really sinks this film is the lack of any plausible motivation for any character involved. For the first 15 minutes, Major Brynner seems to be the good guy, and Long the mad scientist; yet in jail Brynner is somehow transformed into an evil megalomaniac. This conversion is never justified in the film. Meanwhile, Long has supposedly suffered 10 years of terrible guilt over carelessly causing so many deaths in act 1 - but not so terrible that he has stopped developing the weapon responsible. Yet, we are supposed to regard him as the good guy - at least, one minor character gives us a tedious monologue on how wonderful he is. And then, 20 minutes into the movie he abruptly dies (not a spoiler - it's in the trailer), leaving Mason and Arlo (Ulrich and Gooding) to save the world. Confused? Apparently, so were the writers of this nonsense.
Mason and Gooding's motivation for taking on this mission is never even remotely established. Arlo's excuse, that he is threatened with a gun, is discarded about 30 minutes later when Arlo says he knew Mason wouldn't shoot; and besides, what stopped him from handing over the keys to his van and exiting the story? Indeed, the dull scene in the boat where each tells his backstory, supposedly explaining his motivations, is easily the worst in the movie, highlighting the limited acting talents of both Ulrich and Gooding.
Thus, as the film enters act 2, none of the characters introduced in act 1 is alive or recognizably the same person, and the protagonists have only just appeared. Midway through the movie, another key character, Colonel Vitelli, joins the chase. (Apparently the writers missed the class in Screenwriting 101 where you learn not to introduce key characters halfway through.) Vitelli apparently has a history with Brynner, but that seems to have finished up on the cutting room floor, leaving us with a bizarre scene in which Vitelli tells us that he knew all along it was Brynner (so what? that was never in doubt!) and Brynner again confuses us by laying justifiable claim to being the good guy (apparently, he was demoted for protecting civilians in Vietnam). The possibility that Vitelli might have pursued Mason and Arlo in the mistaken belief that they were the terrorists is thus introduced and disposed of in the space of minutes, neatly killing what might have been the only interesting complication in the progress of this story.
Once Vitelli enters, the second half of the movie reprises the same idiotic chases and explosions of the first half, yet no obstacle is allowed to stand for more than a couple of minutes before our two young heroes dispose of it. As a result, the film never manages to achieve any sense of tension or jeopardy.
The film is so riddled with plot holes, starting with a terrifyingly powerful doomsday weapon that can be stolen by 3 people simply by killing 4 guards, that they are hardly worth cataloguing; but it is the character holes, rather than the plot holes, that ultimately sink this movie.