Darren
I built a Mystery last winter and raced it 12 times last spring and summer. My previous boat was a Eric Shade Baidarka S&G. The Mystery is not as stable, but it is not super tippy. I have paddled an Epic V-10 surfski and an V-10 Sport. The Mystery is very similar to the V-10 Sport as far as stability goes. The Mystery does well in chop/waves.
Two things you should consider about the Mystery, first it seems to me to actually be slower than the Baidarka at speeds below about 4.8mph. It is substaintally faster above 5.5mph. This isn't surprizing given it intended use and design. The other thing is that the Mystery is designed to be very straight tracking. Tight turns like buoy turns in races are very tough. Most rivers are not an issue unless they have oxbows with some current running. I actually modified my Mystery's bow and stern at Nick's suggestion and the boat now turns much better.
Mike