Dear Chet--I have two susatos, a C and a low D. The curved windway is lovely because the "moisture" drains away to the sides and doesn't clog the works. I know that unreliability in the upper range is a generally reported problem (I think Dale at Chiffandfipple.com has something about it too). I haven't experienced the problem on either of my whistles, however. This may be because I'm doing relatively slow stuff where precise breath control is easier. My fingers aren't nimble enough yet for the really fast stuff. Also, I jump around from orchestral flute to wooden flutes to recorders to penny whistles, so maybe that has something to do with it.
Good luck--I love my susatos--the C was the only wind instrument I had that would play outside when I was inspired to go whistle at the wind during hurricane floyd.
V