I'm around 380+ now and trying to find a way to take the pounds off. I've been walking (mostly at work) and am getting to a point where I need (and want) to do more. I'm looking to get back into bicycling as a way to do that.
Biking is awesome because it's fun and zippy and you can get a lot farther in the same amount of time -- but walking is also fantastic exercise! Bike because it's fun and adds variety, but I wouldn't expect it to increase your fitness more than spending the equivalent amount of time walking. What I like about the bike is -- well, it's fun. But also, I can ride to the store and haul back a full load of groceries including a gallon of milk and a bottle of wine, and be back home in time to cook, while doing the same on foot would be a major ordeal. Fortunately, there's no reason not to walk at work, and then ride in the morning and evenings or what have you.