I don't know what its cardio value is, but it's a lot better than sitting on the couch and is a hell of a lot of fun. I go rollerblading a couple of times a week at a nearby park and its definitely worthwhile even if it's not the maximum cardio workout you could get.
You can use rollerblading for cardio. I think it easier with a heart rate monitor. I have a Polar one. In order to reach cardio I have to skate pretty dman quick and keep my legs moving the whole time. If I don't it would be anaerobic exercise.