I am of the strong opinion that this show would have been the greatest sitcom ever if Chuck Lorre had not fired Charlie Sheen. The first 8 seasons were great and Aston Kutcher certainly did not have Charlie Sheen's aura and vibes. The show diminished from the 9th season and gradually, I started getting bored.
So, what I'm trying to say is-
Seasons 1-8, The Best Comedy Show Ever!
Seasons 9-12, Ehhh, It's okay.