Because you know you wanna know what I thought of it, right? ADMIT IT!
I suggest, if you haven't seen the movie and plan on doing so, not to read any further as I WILL UTILIZE SPOILERS!!
First of all, ignore all the reviews that claim the movie is horrible. Or just bad. It isn't either. However, it is worse than "Spider-Man" and "Spider-Man 2." As I feared, having too many plot lines, characters, and villains made for a somewhat disjointed flick. The positive is that the film's effects and fight sequences are worth the price of admission alone.
Taking a page from Marvel Comics' "Secret Wars" series from the early 1980s (where several big Marvel characters got some major changes applied to them), Spidey gets a black costume. In "Secret Wars" that was because he, along with many other superheroes, were transported to a far-away alien world. The "costume" -- actually an alien lifeform which needs a symbiosis with another creature -- bonds with Peter Parker, and when the superheroes return to Earth from the planet, Pete brings his "costume" with him. In the movie, Pete and Mary Jane are watching a meteor shower, and unbeknownst to them one of the meteors lands nearby -- and out pops the alien symbiote. It latches onto Pete's moped, and eventually attaches itself to Pete. Like in the Spider-Man comic, the alien "suit" alters Pete's personality, making him much more belligerent and hot-tempered.
Elsewhere, a dude named Flint Marko is running from the police. They chase him into a physics experiment's testing chamber which conveniently activates when Marko is inside. The experiment turns Marko into longtime Spidey enemy Sandman (comic version at upper right) -- a guy who can control all his molecules which have become intertwined with the sand in which he was standing. We're shown that Marko's crime career is supposedly to save his daughter from a health affliction.
Further elsewhere, Pete's former pal Harry Osborn is still planning revenge against Pete/Spider-Man for what he thinks was his father's murder at the hands of the wall-crawler. One night, Harry augments his strength via his dad's old formulas, dons some of his dad's old weapons and duds, and goes after Pete. The sequence is amazing; one thing about "Spidey 3" that is vastly superior to any of the previous films is the choreography and F/X. Anyways, Pete gets the better of Harry, whose injuries cause him to forget anything about being the Green Goblin. To him now, he and Pete are still the best of buddies.
What possibly irritated long-time Spidey fans most was how Sandman -- back before his transformation -- was made responsible for the murder of Pete's Uncle Ben. However, I have to admit it was done quite well; the dude who Pete went after in "Spider-Man" was indeed part of the crime -- he and Marko were both in the process of robbing that wrestling promoter. Marko was the guy who actually carjacked Ben and shot him, but only after the other dude screamed at him to "hurry up and get in the car."
Once Parker learns that Marko -- Sandman -- is responsible for his uncle's death, he goes after him with a vengeance -- a vengeance only made possible by the personality-altering effects of the black suit-symbiote. Spidey apparently kills Sandman by flooding his form with water and literally washing him away. In the meantime, Harry gets his memory back, but when he decides to take on Pete again, Pete's black suit makes him a fighting madman. Pete shows little mercy as he tears into Harry, ultimately turning one of Harry's pumpkin bombs right back at the new Goblin, severely disfiguring him. The climax of Pete's sojourn with the alien suit comes at a nightclub where he ends up hitting Mary Jane who is trying to stop Pete from beating the crap out of some bouncers. He finally realizes that he has to get rid of the suit, and travels to a nearby cathedral to do so.
BUT (there's always a "but"!), photographer Eddie Brock, whose photos Pete had exposed as forgeries earlier in the film, is also at the cathedral (to "pray" for Parker's death, no less!). As Pete struggles to get rid of the symbiote, Brock glances up at the commotion. Eventually, Pete gets the symbiote off of him, but it then attaches to Brock. Brock uses the alien suit to become "Venom" (comicbook version at left) -- the perfect symbiosis since both entities now despise Peter Parker.
Eventually, Venom and Sandman team-up to destroy Spider-Man. But -- and this was a shocker to me as it never happened in comics as far as I know -- Harry decides to help Pete against the deadly duo! (After his butler fills him in on just how his dad really died, natch!) The climactic battle takes place in downtown Manhattan with Mary Jane once again the hostage bait. She's tangled in Venom's web, while it seems Spidey has just about had it (a hugely enlarged Sandman is pounding him with a giant fist while Venom holds him down). Suddenly, Harry appears and enters the fracas. Pete and Harry work together perfectly to hold off the bad guys. However, Venom gets the better of Pete at one point, but Harry sacrifices himself to save his friend! Pete realizes Venom's weakness -- sound -- and begins banging numerous metal pipes together to drive him crazy. One of Harry's pumpkin bombs disintegrates the Venom symbiote -- and Eddie Brock, who was diving towards the symbiote to reattach himself to it.
Pete and Mary Jane tend to a dying Harry, while suddenly Sandman appears. He doesn't want to fight anymore; he wants to apologize to Pete for his uncle's death, explaining why he did it (again, to help his daughter) and stating he realizes it was absolutely wrong -- and wishes Pete could forgive him. Pete does, as any hero truly would -- but then lets Sandman fly away!! For me, this was the biggest disappointment of the movie. What Spider-Man really would have done was forgive Sandman, but demand that he give himself up to the authorities while reassuring him that his daughter would be taken care of. Ugh.
The movie ends with absolutely no certainty of a sequel, but at least Pete and Mary Jane are apparently back together, after Pete's personality changes had driven them apart.