I am absolutely against CIO. My oldest did not sleep much as a baby, and I thought I was going to lose it. I was so sleep deprived, and my husband was working 10 hour days with a 90 minute commute each way. I was so desperate for sleep, and I mention this because I don't want to sound like someone who is against CIO but never had a child that didn't sleep.
What really saved it for me was co-sleeping. If he was sleeping next to me, we both slept. I didn't always sleep with him. I had things to do around the house and really needed to find ways for him to sleep alone. Sometimes I would roll away after he fell asleep. Sometimes I would put him in the swing and let the motion rock him to sleep. You have to be creative, but you can find ways.
If you meet a baby's needs, you teach them that you will always be there for them. They become secure in that fact and feel safe to explore the world. Research has proved this.
If you leave a baby to cry alone, you teach them that you will not always meet there needs. They will never know for certain that you will be there for them. I know plenty of people will say that they had to CIO and they turned out just fine. While that may be so for some people, research (and logic) shows that more often than not, baby's who do not have their needs met grow up to be adults that are less trusting of others and more apt to make sure their own needs are met before the needs of those around them.