Personally, I thought the movie was friggin' hilarious. But I understand that it takes a certain sense of humor to enjoy that sort of thing. Someone mentioned Punch Drunk Love earlier - I think people who liked Dynamite will like that movie - similar weird humor. I thought Punch Drunk was great and Adam Sandler's performance, in my opinion, was worthy of consideration for one of the million awards that are out there.
As for the criticism of the acting in Dynamite, I honestly think that the "bad" acting was part of the humor. It's not that they can't act, it's that the "terrible" acting style contributes more to the goofiness of the characters.
By the way, check out
these pictures of the cast. There are some pictures of what John Heder looks like normally - weird seeing him not look like a loser. Same with Kip. He's actually a decent looking guy.