No question. Shaq is one of the most overrated players in the history of overrated players. And like so many other "greats" of the last 20 years, he only managed to win a title when Michael Jordan wasn't playing.
Shaq, while obviously not as great as Jordan, was a great player. He won 4 titles and for his first 11 years, he was one of the most dominant centers in NBA history.
That said, if he had stayed in better shape and learned to shoot free throws (which any 12 year old girl can do), he would have been much better.
He probably should have retired after he won his 4th title in 2006. That was the last season that he was truly an effective player.
He might help the Celtics a little, though, assuming he knows his role and is willing to play it.