Similarly, I am not a child psychologist but have some little knowledge of development and sleep. As far as possible I try to not let anything I have read interfere with my natural parenting instinct except for in matters of safety and necessity. I hope that I parent in the same way I conduct other areas of my life, namely with respect and patience.
I could never condone cc or cio for my own child and would not feel qualified to comment on anybody else's decision to sleep train any further than I believe that whatever route you follow, it should be pursued safely and consistently.
I can comment only on the child I have been given and the way in which I personally see sleep. I agree wholeheartedly with mamadoc about the developmental nature of sleep. I do not believe that you can train a child to sleep any more than you can train them to walk, talk, eat or reach other developmental milestones. Certainly I acknowledge the need and necessity for guiding them in certain directions but I know for me that her natural circadian rhythms and appropriate zeitgebers such as light in particular will guide her to sleep at the times she needs to. My understanding from research and own instinct is that around the time a child becomes more cognizant of its own body and ability to choose to sleep, it will reach that developmental milestone. At the same time, we reach a stage in our relationship where she is able to communicate effectively her needs. So where in her younger days her only communication was through crying, when she is old enough to self settle she is also able to tell me if any of her needs remain unmet.
As a baby and small child, those plaintive wails or cries were her voice. That was the way she told me her needs and in the same way I would not ignore an adult I loved when they requested my help, it was my job to respond to dd. At no point would it ever have been appropriate for me to allow her to believe that there are conditions attached to my response to her. I could not have allowed her to think that one day I would not respond to her if it was in my powers to do so.
So, we demand-fed, co-slept and dd made the rules. Until she started eating at 7 months, I accepted that her tiny tummy and a milk diet meant she would need to feed often throughout the night. I instinctively knew that after months cuddled in my tummy and listening to my body, she needed to remain close to me and make her own moves to explore her new environment. She found her own way. She sleeps that big chunk at night because that's the way humans function most effectively when guided by light/social interaction/hormones. At around 16 months, she started to self-settle the majority of the time (as research suggested she would).
I absolutely refute with my grand case study of one that co-sleeping eschewers of cio/cc have miserable or whingy children. My dd is the happiest, brightest, well-rested child. I wouldn't dare to presume that luck or genetics aren't involved but I know categorically that the way we did things was the only way for us.