Wheelmap is one of the best examples of an external service using and contributing back to OSM. They have perfected the use of OSM for a simple task - modifying a well-known set of tags on existing OSM objects.
Edits from anonymous users are feeded back in bulk at given time intervals on objects which do not yet have a “wheelchair” tag set. If a user is signed on, he can also modify existing “wheelchair”-tags, delete and create new objects.
The approach of TransforMap should be roughly the same, only our use-case is much more complicated. We have a whole taxonomy instead of three tags, and most of our POIs will be newly added in OSM.