Apparently I'm in the minority that not only loved it, but found it to be the best of the trilogy. Now, before you slam me, I don't think the third film is loads better than the second. I think they're about equal, but the third film gets the edge for more innovative action sequences and (for me, at least) the most interesting Peter Parker story. The quality gap here isn't as great as the leap from the first (which I loved, but found kinda hokey and awkward at points) to the second (which is as close to perfection as I could ask).
The biggest concern seems to be about cramming in so many storylines in such a short amount of time. While this could've been potentially problematic, I think the filmmakers handled it all very well. The film didn't feel rushed or short-thrifted in any areas (well, except the very last scene, which felt like an after-thought). I feel way more satisfied with the balancing of multiple threads here than I did with, say, Crash. That film (which I did like overall) felt very slim when you thought about each character arc and plotline. Each mini story, if edited to be self-contained, would be about 10 minutes. The character arcs in Spider-Man 3 seemed to max out their full potential. Sure, Venom could've had more screentime, but the way the film handled him seemed suitable. In order for there to be more Venom, either Peter's "black Spidey" persona would've had to end much earlier, or the third act would've had to be padded. Considering how well-paced I think the film is, either option would've harmed it.
To make this more well-rounded, though, I'll note things I didn't like about the film:
* Peter re-arranging his hair every time he felt evil was kind of silly. Actually, I wouldn't have minded seeing him with that emo hairdo if we simply cut to him already with it each time. Showing him actually changing it was unnecessary, though. Oh, and as for the infamous Jazz Room scene, this didn't bother me. Raimi was obviously playing this for laughs. Had the intention been otherwise, THEN we'd have a problem.
* Harry's butler suddenly letting on about Norman's death has me mixed. On one hand, it's nice to know that the butler's been awake enough all this time to see what Harry's doing, but on the other, shouldn't he have told him all this, you know, at the end of the first one? I guess I can accept the notion that the butler wanted Harry to hold on to the memory of Norman being a good person, even if it was at Peter's expense. So this really isn't an annoyance with me as it is an oddity.
* Perhaps this would've made matters even more convoluted, but I think Gwen should've been given a role in the climax. After the Jazz Room scene, she just disappears except for a brief appearance at the funeral. Eddie/Venom could've kidnapped her partly as bait for Peter and partly for revenge on Gwen (he probably didn't know the two of them broke up).
* The female news anchor during the climax needed to be cut. Badly. Her commentary was not only superfluous, it was outright cheesy and felt like something from a parody. A guy in my aisle actually laughed out loud when she did her whole, "Oh, no, is this the end of Spider-Man?" routine.
* I would've liked to have known what happened to Flint's daughter. They just left that whole situation open-ended. They could've at least killed her off which, although sad, would at least give some closure to that storyline. In fact, having her die would give Sandman more of a reason to seek revenge on Spidey and would make the final scene between the two of them even more emotional.
* The film's ending was too low-key for me. It just...ended. It should've had a long, continuous shot of Spidey flying through the city like with the first two.
All of these are just little nitpicks, though. Overall, I was very impressed with this film. I do think, though, that they should stop here. Things have been wrapped up too much, and in the context of the overall series, it'd be awkward having a 1, 2, and 4 with linear storylines and a 3 that's stuffed with many overlapping ones.