I purchased a Mysto a couple of weeks ago as the kayak my wife wanted was not available (Venus 11). We have been out 4 times so far on lakes and once in the sea. As for performance it seems very good so far, however once the point of tipping occurs it rapidly empties you out. In the surf it is great surfing in, but heading out against the waves expect some serious water over the bow hitting you in the chest, still you don't expect to stay dry in surf.
Overall performance was good, my main problem was the moulding quality on the hull. The bottom was very distorted and appeared to be dented on one side more than the other. When checked against others it appeared they all had that fault to greater or lesser extents. I had a replacement which was better but far from perfect. Check the hull is my advice.