Cycling, or any kind of cardio can be good. What's important is whether the intensity of the activity fits with your goals. For example, when I run, the limiting factor is my cardiovascular system, or for shorter distances my anaerobic energy systems. So the intensity can be as high as I want it to be. But if someone's not used to cycling, and they don't have the muscular endurance, than that's the limiting factor, so the intensity is lower, and the energy systems aren't challenged enough. Similarly, if someone want to swim for conditioning, but isn't a strong swimmer, then it's their swimming ability that's challenged, not their energy systems.
The point is, cycling can be good, but you may have to spend sometime getting used to it first. Whether you do steady state or intervals depends on your goals. Although if conditioning is a priority, than a combination of the two would be good.
LOL at this. How many calories are burned are dependent on exercise intensity, and time.