Hi Brendan.
We did it at 6 months (son currently 8 months).
Our problems were that he would only ever get to sleep on us even during the day, and would often take 30-40 minutes to get to sleep whilst crying. In the night he would often wake up when you put him in the cot, cue another period of crying...This all made for very tired everyone, especially him and my wife (he would also want feeding every two hours or so including during the night.
We chose 6 months as he had been on solids for a month or so and we thought \ hoped he might also be able to sleep for longer than his previous (and very rare) maximum of 4 hours.
There seem to be different versions of controlled crying, we did the following: solids, bath time, milk, story, put to bed. The put to bed consisted of putting down (he would cry immediately) pat on back for 2 minutes and talk to, then leave him for 5 minutes (we would stay in the room though) then pat for 2 minutes with talking, then leave for 5 then pat for 2 minutes no talking, then leave for 10, pat for 2 and repeat until he sleeps.
First night - 1 hour of crying mixture of pure anger, sobbing etc. At best I would describe this as one of the most guilt ridden horrible hours of my life. Once asleep he stayed asleep til a 10.30 feed and then slept through til around 6.30 the next morning.
Day 2. 20 mins crying. Day3 15 mins crying. Day 4 10 mins crying.
2 months on and he now sometimes falls asleep when crying (we sometimes sing to him instead of the book) but often still cries \ moans for 5-10 minutes, but it more I don't want to sleep crying as opposed to really upset crying.
The one thing I have learned is that different things work for different people.
For us (all of us including our little one) this has worked brilliantly. He is now much happier during the day (he has relatively scheduled day naps as well). Our problem was that once he woke up before he could not get to sleep in the night, he now does, he would only sleep on my wife\me during the day - he now sleeps in his cot.
Good luck - some people hate this and will no doubt think we are devils for doing it - so be it.
My view - our baby cried 30-40 minutes numerous times a day and was unhappy. My wife was exhausted. He now cries a lot less and is a happy baby.
I've probably left something out - if you have any questions fell free to ask!
Once more, good luck!
Al