i bought one of those for my daughter bec, at 2-1/2yrs of age, she was too heavy for the mini-trampoline at the day care (weight limit was 50lbs - they let her on it anyway but i was watching the way the legs splayed out as she landed and told them no way).
she's just turned 5 and uses it *constantly*. still weighs the same but is pushing into a size 8 for the length.
i started using it day bef yesterday and i do the same thing - job and bounce- but to youtube videos (LMFAO, fex). there are rebounder routines right on youtube if you want to check them out.
here's a good article: http://www.how-to-boost-your-immune-...exercises.html