I've given up on making my cat wear a collar. She is an indoor only cat, and she's not the neatest girl, so the two tags that are on it would always get coated with food when she ate, and then they'd stick together and it was really...well, not pretty.
Norville wore a collar for several years, but eventually decided he didn't like it and was not going to wear one anymore. So he didn't.
When I was in college, I was a foster mom for a kitten for few hours. I put a collar on him and he immediately FREAKED out - scratching at it and trying to bite it. Within moments, he got his dang jaw caught in it! I was able to maneuver him out of it, but good grief that scared me! I've always been very careful with collars on kitties after that.