RE: Double fins?
I have ridden the Madd 115 and Madd 165 and a lot of people in my area have the Madd 135. They are nice boards with some major advantages, and no serious disadvantages. I think that twins fins are really good for large freeride boards (115 + liters), especially for manoevers, control, and using a wide range of sail sizes. I wish more manufacturers would make them. The main differences in feel from single fin boards is that twin-fin boards don't rail up as much, and favor more inboard footstrap settings. They go upwind well and rarely spinout or tailwalk, and they adapt well to weed fins.
I don't think they're quite as fast as slalom boards or go upwind as well as formula boards, but they're pretty darn good for freeride boards, especially at going upwind.