Another issue not mentioned (AFAICS) is bike fit - if your bike is significantly too small or not set up properly for you then you will be working much harder than if it fit properly. And since Walmart bikes generally only come in one size, there is a set of people for whom they work, and the rest of us can pound sand. But if the brakes are not rubbing and the tires have enough pressure (around 50 - 65 psi for paved surfaces usually) than even a very inexpensive bike can function properly.