When you say rules of CIO, what do you mean?
I have been consistent with it. I would wait every 10 minutes, go in rub his belly, put his nuk in and leave. But after over an hour of crying, I am exhausted and he is crying so hard I feel bad. Other than this, he is the happiest baby I know.
I'll admit it's been a while since I read the infamous "Ferber book" and that's one I don't own a copy of, but they had recommended guidelines about stretching out the time before you went back into the room - something like 5 mins, 10 mins, then every 15 mins (until asleeep) the first night, then 10, 15, every 20 the second night.
I think where parents really run into problems are situations where they are not consistent with their sleep rules (of whatever method they are using). For example, they are working the intervals, but then eventually cave into the crying, give up and go back in and hold/rock/nurse to bed. This does no good at all because the baby has no consistency (and everyone is tired and miserable after hours of crying). One of my favorite quotes from the Weisbluth book was something like "start as you mean to go on". They don't necessarily favor, for example, putting into crib awake and allowing to fall asleep independently over rocking to sleep, but they do want you do the same thing EVERY time.
I would never recommend leaving a baby until he or she vomits (clearly this would be too upsetting for any parent) and I think many people misconstrue CIO to mean shut the door and leave till the sun comes up. There are lots of books that address independent sleeping in addition to the Ferber book (Baby Whisperer, Sleepeasy Solution, and the previously mentioned Healthy Sleep, Happy Child) and if I recall correctly they all suggest some intervals of reassurance. Point being, if you are OK with letting him cry a bit and are consistent in your responses, know that you should see some positive results. (I believe one guideline many of them use is 12 weeks and 12 pounds as readiness for a full night of sleep).
That being said, I know there are parents that just don't want to do CIO. I was never one who wanted to share my bed with my kids and had seen too many friends with kids who were way too difficult at bedtime and through the night (mom in the room over an hour waiting for them to sleep, up every 4 hours for a bottle when the child was over 1 - the list goes on and on)
I also know (from personal experience) that some babies can be just a bit "trickier" than others. After having two very easy going great sleepers/nappers, I thought I knew the answers. DD had other plans. She just didn't get in the groove as easily with naps, didn't sleep through the night until about 4 months and stirred a bit more when going down.
I think all the posters here can attest to the fact that there are a lot of options and you need to do something that feels right for you. From my personal experience the advice I can offer is that if you're ready to do CIO consistency is key. If you're not ready for this yet and are OK with nursing once a night I say just do it and go back to bed (spare yourself the hour of agony and get some necesary ZZZZZ's). Hopefully he'll grow out of it and just start sleeping through the night on his own.