There's a really nice article on sleep training over at the BBC. It's a long read, but the bottom line is that the research on sleep training effectiveness is sketchy at best. Sometimes short term benefits are shown, mostly for the parents (children still wake up, parents just don't realize). but it's very difficult to remove biases when people's children are involved. There also seems to be a large variance between children and what works for one will not work for all.

The good news is that in the long term it doesn't seem to matter much. Sleep trained or not, a large majority of children are able to sleep through the night at 20 months old.