Myermay, we use it and ds is only 17 months. I've read Christopher Green and we've both watched Little Angels.
We've used it once each. I used it one evening when nothing was right (I love Green's quote that "toddler's don't know what they want, but they want it now!". DS would reach for his milk (etc.) and when I reached for it he would bat it away. He was having tantrums and doing that wilful squirm that toddlers are so good at. I tried distraction (I was only trying to get him in the bath), I tried cuddles etc. So in the end I picked him up ery calmly but firmly, looked him in the eye and explained that I loved him but he was going to take some time out to calm down. I put him in his bedroom and shut the door. I then waited exactly three mintes. When I went back in he greeted me with open arms and gave me a huge long cuddle and I gave him a huge long cuddle back. And then the bedtime routine went like a dream.
When dp did it, ds was repeatedly doing something he shouldn't (probably climbing on furniture) and ignoring or laughing at "no!". DP did exactly the same, and again it had a wonderful calming effect.
Since then (c. 4 weeks ago) we haven't used it again, but we feel really confident knowing that it is there for us when we need it! If you think your son is able to deal with it, I'd give it a go. Feel free to ask any more questions.