Last year I bought an Osprey Vipermax 7. I'm very pleased with it. It's my fourth rib. Sailcaracteristics are superb! At first I had some problems with compensating the propellor-torc but with placing the engin an inch to the sb-side that was solved.
Now the only thing I don't like is the fact that with low speeds, when not in the plane, the rib goes any direction, accept the one I want to go to. I'm constantly steering to keep the right course. Perhaps trim tabs could help solve this inconvenience. I think it's the result of cutting a triange-shaped part from the hull / transom that Osprey does to improve speed.
With the mounted Yamaha 200 hpdi vmax I reach 50 knots, with two people on board and not to much fuel in it.
Average I burn 5 gallons per hour. That is in fact economical but if you'r out a whole day still a lot of money! Specialy with the fuel price nowadays.
So, (if there is a) next time I would go for a diesel engine.
For pictures you can go to http://www.ribadventures.TK