It was okay, but it really was clogged up with too many characters and too many subplots. I would've kept Venom and developed that storyline a little more, and completely left Sandman out of it.
The thing I liked (well, loved) about the first two movies was that each had a theme that was well developed and firmly rooted in the comics. SM3 tried too hard to force the whole vengance/forgiveness theme, and Sandman seemed shoehorned in just to make that theme all the more explicit.
The same theme could have been more effectively carried out simply between Harry and Peter, and the Venom/Eddie Brock subplot could have been used as a counterpoint if it had been developed a bit more. The elements were there, anyway (like Eddie vowing revenge for Peter wrecking his career, etc.)
The retrocharacterization stuff that generally wrecks sequels was completely missing from SM2, but was relied on too heavily in SM3. How convenient that Sandman was retroactively written in to have been the cause of Uncle Ben's death, and how much more convenient that Harry's butler finally comes clean about cleaning Norman's wounds and whatever. Too convenient, too contrived.
I did like the bad Peter stuff, and Bryce Dallas Howard is ******* hot. If they do continue with this series, I hope that MJ gets killed and Peter marries Gwen instead.