Went to a demo day yesterday and tried a few boards. It was my first time SUPing, but the second board I tried was a 14' Bark race board and it felt fine -- no stability problems(on flat water). Speed and tracking were much better than the novice boards. The more surf-oriented boards felt sluggish on flat water. One other board I really liked was the 12' Bic Ocean, which felt like a nice blend of speed and maneuverability.
At first I tried stern rudders for steering, but realized that I was fighting the fin. Bow rudders were much more effective.
I am thinking about adding one to the family fleet. It doesn't replace the canoes or kayaks, but is another way to have fun on the water.