(I am not a Twilight fan and not in that demographic, so I am reviewing this just as a regular movie.)
They underprepared and he overreached. This could have been a pretty solid, derivative action movie. Unfortunately it was marred by a script with numerous logic problems and occasional wince-inducing dialogue. Sigourney Weaver looked embarrassed to be there, and Lautner is still too stiff and inexperienced an actor to rise above the mediocre material. (And how DID the motorcycle survive the exploding house? That must be one tough garage it was in....)
On the positive side, Lautner did fairly well in the action scenes and appears to have definite potential as an action star if he works at it and mentors with more-experienced actors. Some of the supporting cast, particularly Alfred Molina, gave creditable performances for what they were given to work with.
This is probably best watched as a rental, but if you keep your expectations modest and just look for some decent action sequences with an up-and-coming actor, you should do okay with it.