I'd say it's like anything to do with general fitness, it is only as good as the time and effort you put in.
It's a good starter for anyone looking to get into exercise / weight loss, but doesn't want to go to the gym or venture out into the cold. It's cheaper than a years gym membership. Although it's not a full replacement for Gym / outdoor exercise.
This is from someone who used to go to the gym 3-4 times a week for several years and now is a complete slob.