DrtBlvd a user here, has an interesting view on this movie and one personally think and can stand behind too. I hadn't read anything about this, neither had I seen the trailer. So I had no idea what this would be about. I actually thought based on the cover this would be something along the lines of "School of Rock". It couldn't be farther away if it tried (though I can't say for sure if it wasn't the intention anyway to be as far away from a comedy like that in the first place).
Jack Black has some of his usual shticks, but it's more than that. It has themes about self identity, about growing up and especially about being who you are and not seeking being someone you are not. In the end we are all heroes (in our own story). Maybe it takes a movie like this to realize it for some, maybe some find it redundant. Whatever the case, this is entertaining, but also raises some interesting thoughts and questions ... but yeah the ending was a bit rushed ...