Baby will feel insecure if leave for crying alone. I will never try CIO when baby is young. But when baby is older and able to understand what I am saying, I will just tell him/her it's time to sleep and you should sleep in your bed. Yes there will be some crying but that is the way the kid let out the emotions and after few cries they will stop. Thus, if you using CIO never leave baby alone but to stay beside him/her.
My 6-month-old was waking every two hours at night. I was rally very tired so one day I just let her cry out. But the next night she slept for 10 hrs and woke very cheerful and rosy. It's been that way ever since, and she's even a better napper now. I know that it is hard to listen to your LO cry, but a sleep-deprived, miserable mom and baby is a terrible thing too.
My son was never out for cry it out, we sung, walked and drove him to sleep and later used to read out a story to him at bed time. But with my younger daughter I decided to try CIO and it worked. At 12 months, she goes to sleep at night by herself and never cries.
thanks