I liked Ed Norton, he was perfect as Banner. The only problem with the Hulk movies, is that the Hulk himself always looked naff. You cant have the movies title character look that rubbish and unbelievable.
Juries still out for me on Thor and Captain America. I can't imagine Thor will translate well (especially since its based on the mainstream Thor, and not the Ultimates version) on screen and Cap.. well, he's a guy that can throw a shield really well. And we've already seen the best use for that shield, courtesy of Tony Stark.
With regards to Spidey, I think Tobey over-estimated his 'draw'. From what I understand he'd been on and off about a fourth movie and they finally decided to just recast the whole thing. While the spidey movies were great, it'll be interesting to see a new take on it. While they got a lot of things right, they also did a lot of things wrong (particularly Venom and Mary Jane). I for one will be glad to see the back of Kirsten Dunst as Mary Jane lol. Tobey was pretty good, but I don't think he's quite as Iconic in the role as others have been in their respective roles. For example, you couldn't imagine anyone other the RDJ as Tony Stark/Ironman now, or anyone other than Hugh Jackman as Wolverine. Or anyone else in the role of Blade than Wesley Snipes. There are a few others that are just as iconic in their roles (Ian McKellen is a fantastic Magneto, and Patrick Stewart is a fantastic Xavier), and I just don't think anyone in Spider-man had quite the same impact. With the possible exception of Alfred Molina as Doc Ock. And I don't think anyone else would have done quite as well as Willem Dafoe as Norman Osborn/green goblin, although the costume was horrendous. Franco was good as Harry Osborn, and he had the right chemistry with Macguire to make their friendship seem real on screen, but I don't think either of them were iconic. But thats just my opinion really.