The pilot stated that he was in the left seat and a pilot rated passenger, who was a certified flight instructor (CFI), was in the right seat. The flight instructor stated that his seat was positioned aft such that he could not reach the rudder pedals. The pilot positioned the airplane on runway 35 for takeoff and said that he rotated a little bit early (70 knots) during the takeoff roll but it was manageable. The flight instructor stated that the airplane swerved left and right before the pilot rotated. The flight instructor called out to bring the throttle back, and described the remaining portion of the flight as porpoising. Both pilots reported that there was confusion in the cockpit. A witness said that he observed the airplane rotate early, enter a steep nose high attitude, and then begin to settle while the elevator was moving up and down erratically. After skidding for 497 feet the airplane went off the end of the runway and into brush causing substantial damage to some fuselage bulkheads and the right wing spar. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.