i've used one for lungeing in the past, never riding. how can it possibly teach the rider and horse about contact? the moment you take it off, most horses will go straight back to where they were, i reckon.
I used to use one to help encourage an outline but never tight to 'force' however... after a while all it made him do was shorten his neck and bunch up so that he could lift his nose up...if that makes sense!! sorry I know I will get shot down for this but I am using draw reins at the momement as I am working towards a topline for showing. whichever you decide to use you need to make sure you really push them forward to make sure they are really working from behind and not in a 'false' outline.