The city of Los Angeles finds itself protected by an honest-to-god super hero with the unlikely name of John Hancock. Unfortunately, this hero causes more damage to the city than he prevents due to the extreme lack of due-diligence expressed during his slapdash crime fighting activities. One day Hancock saves a mild mannered PR consultant named Ray. In return Ray uses all the tricks of his trade to redeem Hancock's image in the eyes of the public, over the objections of Hancock himself (who sees little reason to change) and Ray's increasingly skeptical wife.
From the preceding description (and from the trailer), you may be forgiven for thinking Hancock was another action-comedy comic book spoof. Already you are probably predicting how the hero will try to change his ways, failing at first but learning from his mistakes - eventually overcoming both his own uncertainty and whatever evil villain threatens the city in the third act, helped alone the way by the unwavering trust of a small child. Hancock contains all these elements, but manages to avoid slipping into cliché by having the plot take a major left turn about a third of the way in.
To say more would spoil it, but suffice to say that Hancock manages to become a more interesting movie. I won't say Hancock is an instant classic - it still has problems, but there are plenty of worse films you could waste your money on.
Recommended if you like this sort of thing.