LOL at Grayland..... I think there's nothing wrong with ellipticals per se as a cardio exercise. It is a low impact exercise much like a stationary bicycle. The only problem I can think of is that training on elliptical does not necessarily translate to running. In other words, getting very good on the elliptical will not make you a better runner in my experience. So in that sense, elliptical training is perhaps not the best choice for functional fitness. But that's hardly surprising. If you want to be a better runner, you need to run. If you want to be a better cyclist, you've got to get on the bike. These activities are different enough that other than helping you get to a certain level in cardiovascular fitness, they don't necessarily translate from one to the other. Since the OP is replacing a treadmill with an elliptical, the question to him is whether he is targeting the elliptical to help him train to be a better runner. If so, bad idea. If he is simply after a reasonable cardiovascular exercise, then I don't see anything wrong with it.