Well, foil surfing looks cool, and the obvious question is: what's the speed potential? The reduced surface of the foil should theoretically allow for great speed. The AHD video seem to focus on speed at low winds. Some web pages mention minimal power in the sail while the board is riding on the foil, also indicating great potential.
However, there's one potential problem with the foils. On a conventional windsurfer, it's possible to go faster by reducing the water line and the surface area in contact with the water. A formula board at high speed will ride pretty much only on the fin. Yes, dedicated speed boards are very thin, too, but even wider slalom boards can reach amazingly high speeds through proper shape, rocker lines, and tricks like cutouts. For the foil windsurfers I have seen in videos so far, there is a problem: the foil is entirely in the water, and surface reduction is not possible. If the foils gives enough lift to get going in 15 knots, it probably would generate too much drag and/or lift to go 50 knots.
Here's a simple proposed solution: a multi-foil windsurf board, with maybe 2-4 foils at different depth levels.
At slower speeds, all foils are in the water and provide lift. At higher speeds, as the board rises out of the water, the upper foils are in the air instead of the water, minimizing drag and increasing top-end speed. A larger number of foils (perhaps 3-5) could have the same effect as multiple gears in the car - go faster in higher gears. Of course, the foils may be as fancy as the little airplanes on the AHD AFS-1.
An added bonus of the multi-foil concept could be that the individual foils could be smaller, and therefore less dangerous. The lowest top-speed foil on a speed board could potentially be really small. The hydrodynamics of such a setup are not trivial, but I think it's worth trying out. Maybe 5 years from now, the water speed record will be held by a foil windsurfer, not a kiter :)