Roller skating is great exercise!
It can be a very effective cardio workout, but it will also help tone up your bottom and thighs. To get an even better toning workout, try to incorporate some larger side-to-side glides like you see those speed skaters do on TV.
Some of the best and most effective workouts are the ones that are "fun", that you enjoy for their own merits on top of just being good exercise.
Good luck!