Canon 100-400mmL Push-Pull

I am deciding on either a 400mmL f5.6 fixed focal length or the 100-400mmL zoom. I understand that the 100-400 zoom is push-pull instead of twist. I once had a Sigma push-pull zoom lens. I didn't like it becuase it was difficult to get exactly the framing I wanted. Can anyone tell me about their experiences with the 100-400 push-pull?

How about the 400mmL fixed, any one have experience with that lens?

Al