We didn't use CIO with our son, but I know many parents on this forum and on the facebook group have. There have been mixed results. Some people have said their baby cried for hours and hours, and still never learned to sleep. For others it has worked well. For our son, we never did it (at least not for more than a few minutes) because my gut told me he would get too upset and it wouldn't work. He still, at almost 4, does NOT like being in his room with the door shut. Even for time outs, he has at times gotten so upset he almost throws up.
We did CIO with our daughter, and it worked. But again, she's a much different temperament.
There are other methods you could try first, if you felt like you wanted to. I think sometimes CIO is the answer ...especially if you feel you're suffering physically and emotionally. When Sammy was older, we taught him to fall asleep alone by sitting next to his bed holding his hand, then the next night moving further away, etc. That might only work when they're a little older, but you could always sit/lay next to his crib so he can see you?
Sorry, I'm probably not much help, but those are my thoughts for tonight lol
