Have one in progress on my G17. Looks like the way to go for a couple of good reasons. Flat rudders stall at 5-10 degrees of deflection, and NACA foils stall at 10-20. Most kayak rudders will easily turn 90. Once you lose laminar flow, you have pure drag. Also, Mariner Kayaks says that a rudder costs 10% drag at 3 knots, even straight ahead.

I built a mockup plywood skeg, 9" x 4.5" triangle, and temporarily hot-glued it between Forms 13 and 14. This straightens things out really well, even in high quartering winds. I'm halfway through fabricating and installing a retractable of the same size, so email me in 2 weeks or so for an update.