I actually decided to watch SM3 recently due to this thread! And I'm surprised to say I really enjoyed it. It's definitely a flawed movie and has a lot stuffed into it, but I think it makes the series feel like a complete story by resolving the Harry subplot running along all three films. Plus, the way it ended up being a trilogy with Spider-man's 3 biggest foes (Green Goblin, Dr. Octopus and Venom) is kind of neat.
If Raimi's Spider-man had some more jokes, then he'd definitely be the perfect film incarnation of the character. As it is, Maguire is the best Peter Parker for capturing his sheer dorkiness and his more selfish side (which Holland's Parker really lacks) and I love that Harry Osborn is such an important character throughout the movies.
I know a lot of people said that Venom's addition brings the movie down, and it's true that Raimi didn't want to include the character, but I think he was kind of necessary. I can't see Sandman being the climactic villain for Harry and Peter to team up against, and I really liked seeing those two team up. Plus the scene where Peter uses the metal poles to exploit the symbiotes weakness is really good.
Sandman's a much easier character to cut, even though Thomas Haden Church is excellently cast as him. It's true, the plot can run just fine without him (the movie starts by focusing on Peter vs Harry, then Venom comes around after Peter beats Harry and they team up) and I don't like the obvious retcon into Ben's death... but I still think I prefer him being there. A lot of the scenes he had were really good (like his forming and the deleted sandcastle scene) and he's important to the theme of revenge with Peter initially being consumed by revenge and learning to forgive him (mirrored by Harry's desire for revenge). Plus, I think it's important to have two villains for the final fight to suitably increase the stakes where Peter really needs Harry's help, as well as have a lesser threat that the two can successfully beat before Harry's sacrifice. The way Peter/Harry worked together to beat Sandman was super sweet.
Personally, I'd just change it so that he didn't accidentally shoot Ben (that's a weak resolution) because I think it works pretty well for him to shoot Ben because he THOUGHT Ben was mocking him instead of trying to help (he says as much in the film, but that's just not the reason he shot him). Then add a scene at the end where he turns himself in after Peter forgives him, and I think that's a better resolution.
Also watching this made me realize, for all the flak that MJ/the relationship writing gets, I don't mind her role either outside of getting damseled. Prior to that, she's got her own goals and we see her relationship with Peter from her perspective. It also helps that in the relationship problems they have, we can very easily see how Peter is at fault, and it's understandable that he acts that way when everything goes to his head. Just cut the part of MJ being damseled again - it'd make just as much sense for Brock to go after Gwen, giving that he was kind of obsessed with her and it could be one of the things he blamed Peter for. Or even don't have a damsel at all! Venom could just publicly threaten to expose Spider-man's identity if he doesn't come to the confrontation.
Then MJ can go and be the one to convince Harry to help Peter, similar to the editor's cut where it's looking at an old photo of them that convinces him rather than his butler. With that said, I don't really get the hate on Bernard telling him that Norman Osborn died by his own hand. The classic complaint is that he didn't tell Harry sooner, but there was no indication that Bernard knew that Harry had found out Norman was the green goblin or become the new goblin (nor any indication Bernard knew Peter was Spider-man). It makes sense for him not to tell Harry that to try and keep him in the dark and protect him (admittedly, the movie should have made that clear).
All in all, the movie could have been made a lot tighter, but I still really enjoyed it. I wish Raimi had been given a proper timeframe to work on Spider-man 4 so he didn't quit the series, it would have been cool if Raimi's Spider-man got to be part of the MCU as that would be truer to the comics with him as one of the first heroes and more of an independent character.