THE ACFT FLEW TO THE ARPT FOR A SALES DEMO FLT. THE CREW BOARDED THE ACFT AND TAXIED OUT FOR DEP. WITNESSES, INCLUDING TWO PLTS WITH AEROBATIC EXP, WATCHED THE ACFT DEP, MAKE A LEFT TRAFFIC PATTERN AND DO A LOW FLY-BY DOWN THE RWY. AT THE DEP END OF THE RWY, THE ACFT PITCHED UP INTO A STEEP CLIMB. AT 600 FT AGL, THE ACFT ENTERED A LEFT AILERON ROLL, WHICH THE WITNESSES SAID WAS 'SMOOTH, COORDINATED AND WITH THE NOSE ON THE POINT.' AT THE INVERTED POINT OF THE ROLL, THE ROLL CHANGED FROM AN AILERON TO A BARREL ROLL. ONE PILOT WITNESS SAID THAT IT APPEARED THE 'CREW LOST IT AT THE TOP' AND THAT THE CREW 'HELD THE BACK PRESSURE TOO LONG AT THE TOP.' AT THE 270 DEG POINT OF THE ROLL, THE ACFT WAS SEEN TO 'FALL OUT' OR 'DISH OUT' OF THE ROLL; IT RECOVERED TO WINGS LEVEL FLT AT ABOUT 100 FT AGL IN A VERY NOSE HIGH ATTITUDE SETTLING INTO THE GROUND WITH A HIGH VERTICAL DESCENT RATE. NO PREIMPACT ENG OR CONTROL SYSTEM MALFUNCTIONS WERE FOUND.