I finally got around to watching Spider-Man 3 the other day... I avoided seeing it until now because it didn't look very good to me, and none of my friends had anything good to say about it.
I guess it was watchable, but I can't really recommend it. There were too many villains and not enough time spent on any one of them to properly develop the characters or explain what was happening. The three main villains were Sandman, Venom, and the New Goblin... yet I think Sandman is the only one of the three to ever get called Sandman by anyone, and that was only one time. Peter Parker refers to Harry Osborn as Goblin Jr. at one point, but I don't think the name New Goblin appeared anywhere except in the credits. And I never heard anyone refer to Venom by name either.
Sandman's origin is not explained very well, and the sudden attempt to include him in the murder of Peter's uncle came off feeling a little too contrived and clumsy... and the conversation about forgiveness he has with Peter at the end of the movie ends up feeling a little too anticlimactic.
Some alien goo became a new suit for Spider-Man for a while, until he decided it was making him a douchebag so he found a way to get rid of it... then the goo attached itself to another douchebag named Eddie Brock and became Venom. They had a scene where Dr. Curt Connors tried to explain the nature of this goo to Peter, but the whole scene seemed to be played more for laughs than anything... it didn't come across as very believable. Also, the goo's mind-reading abilities are only hinted at, never really addressed directly.
Harry Osborn was out for revenge from the get-go, which was to be expected given everything that happened in the first two films, but in this one they had him bouncing back and forth so many times from bad to good to very bad to very good... it got a bit tiresome.
Another problem was that Peter Parker himself becomes a bit of a villain for a while before he finally sheds the black suit... well, he actually started acting like a bit of a douchebag before he has the black suit. I was really annoyed by the attempt to make "bad Peter" look cool by giving him a really stupid emo hairstyle. I was also annoyed that he never said a word to Mary Jane about the black suit, he just let her think he suddenly became the world's biggest asshole for no reason... I know they were trying to make a point about forgiveness and owning up to mistakes, but come on.
I really wish the same writing team from Spider-Man 2 could have been brought back to write this movie too, I think it would have turned out much better. Even after stumbling a bit with this movie, they still could have brought back that team to write a fourth movie, but for some reason they tried using some completely new writers... nobody could agree on a script, and the whole thing fell apart when Sam Raimi backed out. So now we will have a stupid reboot instead of what could have been a decent sequel. Oh well.