In short:

Spiderman - great, but weak villain
Spiderman 2 - great, better villain
Spiderman 3 - complete mess, and a lesson in what happens when you chuck too many villains into your films (as well as what happens the studio start forcing the director to do stuff he doesn't want to do)

It'd be a lot better if they just let the whole thing stop rather than further mess up the franchise, but while there's money to be made that'll never happen. Mind you, it can't be worse than Superman IV or Batman and Robin surely...