Bikes are generally not as good as other forms of cardio, because the body's not supporting itself nor are the arms moving, but can work if the intensity is high. If the intensity's not high, biking's a good supplement to more effective forms of cardio. Spinning classes are an example of effective bike cardio-the intenity is very high, HIIT for biking.
Recumbent bike's use even less of the body's resources than regular bikes, are in the "ok" category.