It's bullshit. Don't listen.
SPIDER-MAN 3 is the best of the three films in the series. There are a couple of slow bits at the beginning, and a sequence in the middle that may or may not work for you. It worked for me. Spidey's dark side is, when he's Peter Parker, a dork side: he goes about looking emo and goth, and the effect is at first off-putting and laughable, because you can't take him seriously as a "bad guy." He's not so much Peter Parker as Pete Wentz (and JESUS, I can't frigging believe I know who Pete Wentz is; Zee, I BLAME YOU). But while it's a freakish tonal shift, it also makes sense in a bizarre way. Look at your lead actor: he is *Tobey Maguire.* He is not going to look dark or threatening or anything. So the film portrays Peter's idea of what dark and threatening is, and it turns out to be a totally dorktastic version of dark and threatening. It works in the weirdest way, but I can see how it would throw some people off.
Anyway: movie=awesome. James Franco again gives the best performance of the younger cast members, Thomas Hayden Church is great as Sandman, and the visual effects animators do some of the best movement stuff I have ever seen.