I do Spin/RPM/Cycle classes regularly (4 - 5 a week).
LOVE it, highly recommend them to anyone. Hated the first one was addicted by the third
Its been awesome for my legs and bum and even my stomach and triceps and fantastic for cardio strength/endurance. I wear my heart rate monitor and can peak it up to 95% max rate on the high intensity tracks. In a 45 minute class I burn between 450 and 550 cals. Your instructor is probably pulling your leg on the 2000 cals, unless they were talking kilojoules lol. But even so I know people who've burnt up to 700+ or so in a class (again, HRM).
Good luck!