This movie is getting pretty heavily panned by the critics and I don't know why. Sure, they crammed an awful lot into two and a half hours and deviated from the comic book version a bit and made Peter deviate from character a bit, but other than that it's pretty good.
You can't give Sandman a whole movie to himself. He's no super villain. He's not out to destroy the city or take over anything. He's just an average crook who got himself some powers. He's more likely to be someone's henchman than the guy plotting something. In the comic books he eventually reforms and becomes a hero.
They got Venom mostly right. In the comic books he was discovered in an alternate dimension during a massive crossover between most Marvel comics. While I'd love to see a crossover movie this isn't the story to do it with. Instead he just drops from the sky on a meteor.
In the comics once Peter was free of the costume it was held by the Fantastic Four to study and then escaped. In the movie they had Prof. Kurt Connors (who we're still waiting to become The Lizard) playing the role that Reed Richards played in the comics.
In the comic Harry Osborne finds his dad's Green Goblin gear and tries to kill Peter/Spiderman. Eventually there's a huge fight which wipes Harry's memory. He goes on to marry and have a little boy who hates Spiderman with a vengence. In the movie [spoilers omitted]. If you've seen the first two movies then you know what's coming.
Peter manages to screw things up with MJ just by being clueless and self-centered. Then others help screw things up more. If you're one of those people who hates the relationship angst part of the Spiderman movies then you'll want to skip this movie.
While using the Venom suit Peter's powers are amplified but so are his emotions. After the nasty breakup with MJ he gets Gwen Stacy and goes to the jazz club MJ works in to mess with her in a scene much like the jazz club scene in The Mask. At this point in the movie he's also switched to emo hair which is more comical than brooding.
Gwen Stacy was the love interest in the comics before MJ. Remember that scene in the first movie where Green Goblin has MJ and a trolley full of people held up on a bridge and makes Spidey choose who to save? That scene was lifted from the comics but with Gwen instead of MJ. Spidey caught them both, but the web made Gwen stop too fast and killed her.
There was a bit of back story fixing to make the Sandman story fit. I kinda wish they didn't do that, but there was a message that the author wanted in the movie. The message of the movie is that revenge is bad. Harry's need for revenge is driving him mad. Peter's need for revenge turns him dark and brooding with bad hair. Eddie's need for revenge makes him into a villain. Ahab's need for revenge killed him and his crew. Blah, blah, blah.
For the most part it's a good movie. It tries to be deeper than the first two and ends up being not as good, but it's still better than most superhero movies.