I had a medium out for a weekend on the ottawa and lachine. I think the m was a bit to big for me and i was definitely sitting a little lower than I'd prefer, but... In holes, it loops like a mofo (huge pop) and swaps ends really smoothly. On waves it moved around a faster than I would have expected -- super fast down the face with big 'ol bounces into whatever your trying for. I found that i was having to muscle it from one edge to the other to really get it to snap around, but I'm sure thats becuase I am on the small side of that design intended range. It seemed like a pretty good river runner too, but the only river characteristics i had to judge that by were big mushy wave trains.
The only thing I wasn't crazy about were the adjustable outfitting which seemed to loosen up and slide into the wider position. If there were a s/m size I'd probably have gone with that boat.