I used it an awful lot on DVD's, but not so much on TV (Almost never actually). I guess DVD's are so geared towards surround sound that all the other noises drown out the dialogue and it's often difficult to understand what the actors are saying. So, I usually just leave on CC.
Also helps if folks are chatting or making other room noises drowning out the tv and it'd be 'rude' to overpower them by raising the volume.
I sometimes wish the theaters had it too so when stupid folks clap at the screen or make other noises around you, you can still hear what folks are saying. Well, that and there's no rewind for when you CAN hear what they said but the accent / gibberish is too difficult to comprehend.