I haven't bothered to see it yet, but I hear Transformers 3 is pretty painful to watch.
It is. It's terribly boring, and it falls flat in just about every category. But it's not quite as bad as Revenge of the Fallen, thankfully.
Can't really add anything that hasn't been said in this thread, tbh. Matrix Revolutions, X3, Spider-Man 3, POTC 3.... can't say that I liked any of 'em.