We did it with DD when she was about 6 months and it worked after 4 nights. We have since been able to put her in bed and walk away and she falls asleep when she is ready without crying and sleeps all night.
We tried it with DS and it didn't work, NOTHING seemed to work, well slightly different things every few weeks.
I still really don't know whether I think it is right or not to do this even though we did it. At the time we thought this:
There are 2 options:
1 - we spend loads of time trying to get her to sleep and don't get a rest ourselves and therefore get irritable from our own tiredness, resent doing it and get irritable, then feel generally irritable with her and she picks up on this and feels unloved, or
2 - do controlled crying for a few nights and feel like utter b**tards but then she sleeps without crying every night after that and we get a break and are less irritable, so basically we are all happy for more of the time than with option 1.
We aren't so 'saintly' that we could spend hours trying different things to get her to sleep while we became more tired and got no break and do this WITHOUT feeling at all cross and irritable - anyone who can do this, I admire you, how do you be like that?!
We did it by strict timing of 10 mins at a time before going to her and no longer. 10 mins seems like hours when your child is screaming and I would have found it impossible sometimes if DH hadn't physically restrained me from going to her before 10 mins! I got really angry with him but in the end the technique paid off. I think it is an excellent idea to have someone there for moral support because it feels so horrible to let your baby cry for any length of time. It feels unnatural - which is why I can't be sure it is right to do it or not. I just feel I would have been a really moody horrible mother to her if I had not done it and that seemed worse.
As they have got older DS's behaviour (who we didn't do CC with) is no less challenging than DD's although they are challenging in different ways, and they both seem generally happy.