I thought the kid that played Jacob for the movie was just like the "kid" in the book. Remember Twilighters, in Book 1 Jacob is suppose to come off as immature and young teen, not the masculine player he becomes in book 2 and onward. I know it's hard to "think back" that far for most of us... but he start out kinda wimpy.
I think the movie was OK. I am sure after the turn out to the first one, the new director will include MUCH better acting. Remember how the Harry Potter movies and kids grew with the storyline... I think we will see much more mature actors next time around.