Ford,
Bicycling isn't a weight-bearing exercise, so it won't have the same weight loss benefits as running. But, as others have said, it's MUCH easier on the joints (for the same reason.) And it can be used as PART of a weight-control program. Of course, if you ride so much you're physically incapable of eating enough to keep up the calorie burn, I guess that would work, too. For reference, I use a rough figure of 6000-8000 Calories per 100 miles, depending on how hilly or flat the route is. I personally have a hard time eating that much, so riding back to back centuries usually results in weight loss. That's a lot of time in the saddle, though!