I know I've made this point before (and it's certainly not original with me), but here's the problem with blockbuster movie series: they can run out of interesting things to say about the characters, and instead become going-through-the-motions exercises all about shaking down the rubes in the sticks for more cash.
So how about this one? The good news, I suppose, is that it's not as bad as it could have been. But it's clearly a couple of notches away from the insane greatness of the first two entries in the series. (Tomatometer ratings for the three movies: 90%, 93%, 67%; IMDB user ratings: 7.4, 7.8, 6.7.)
So, something's missing. The romantic troubles between Peter Parker and Mary Jane Watson seem contrived. Aunt May is reduced to a tedious moralizer. The villains—many more than there should be—are nowhere near as clever or menacing as Doc Ock or the Green Goblin; they're just strong and near-indestructible. The wit is gone, replaced by some funny stuff from J. Jonah and Bruce Campbell (who's had cameos in every Spidey movie).
But, hey, it's Spider-Man! So if you're like me, you'll line up dutifully, even if only for the DVD.