I really liked it. Far different than I expected (when I saw the trailers I was thinking like a pretty much straight up superhero comedy, but it was a lot darker at some points than I expected). I loved Smith's character, how he wasn't a typical save-the-world hero.
I agree, the twist was kind of dumb, but I don't think it bogged down the movie too much.
It was certainly enjoyable, and that's really the only thing I could ask for. Plus, Jason Bateman is hilarious.
EDIT:
it really reminded me of the tone and plot shift in click
I thought the exact same thing while I was watching it. I thought it was handled a bit better than Click, however.