Very enjoyable flick. I think it's getting bashed for unfair, but understandable reasons.
Yes, the origin story is sketchy. Any sketchier than say... a genetically altered spider overnight changing the DNA of a human being through one bite? Not at all, but Hancock, being an original character, doesn't have the 'okay we're going to go with this' acceptance factor other superheroes have built up over time.
Tonal shifts? More abrupt than Superman: The Movie? Not even close.
Inconsistent powers? Seriously - someone's going to criticize a superhero story for that? That's so common it could be considered a convention of the genre.
So, being as Hancock is quite normal for a superhero story - what are it's real strengths and weaknesses?
Smith is fantastic, and carries the day, especially hard when establishing a new superhero on film. He's funny, poignant and heroic.
Bateman is showing his usual understated charm. Ray's arc is as compelling as Hancock's, which is nice.
Theron does a great job of playing her repressed feelings for Hancock, and stands up well as a superheroine herself.
For me the movie's biggest flaw is in filling in the backstory and in the final act of Hancock's heroism, which still played well, but lost its emotional impact upon the slightest reflection. What should have happened was that during the fight with Mary, Hancock regains his memory, and has a series of flashbacks to explain their past together. Not only would this have been some interesting action and better explained the whole 'the more time we spend together the weaker we get' issue, but it would have kicked his leaving up by a power of ten - because he would remember loving Mary, but would leave anyway so she and Ray and Aaron could have their family.
Somebody above mentioned not understanding how Mary and Hancock could have lived 3000 years - but she explains that. They are continually driven apart by attacks, which breaks her heart over and over again. That's why she wants to stay away from him. She knows it will happen again.
What no one seems to be talking about, which I just can't believe - is the allegorical aspects of this movie. Smith, I have no doubt, is quite aware of them. The entire first half of the movie struck me as allegory about the choices facing men, and specifically black men, as they grow up in our society. Do I indulge my power in quick and easy ways, take what I can get just because I can get it? Hardly a new theme, but, as always, seeing it played out larger than life in superheroes is very powerful. When Ray talks to Hancock in prison about being ready when they call because they need you, it made me think of all the guys in prison and their families. Maybe this is because I have a step-brother who just got out after 8 years inside, but the issue of the lack of a black Superman has long been discussed. To have one realized in this way - it's very in touch with real issues in contemporary society. Is it deep? No, but it is right there with the rest of superhero history - reflecting wish fulfillment fantasies and growing pains.