I do cardio by jumping rope twice per week for twenty minutes a piece. Is this enough cardio? And what about longer term impact to joints and such, is it better than running?
Enough for what? What is your goal?
I think people with knee problems has reported here before that jumping rope is ok for their knees but running is not.
Check in threads page 51-101,"Jumping jacks bad for knees?" thread for some info on rope jumping.