They can be good for smaller yards with certain types of grass. I wouldn't go mowing an acre of St. Augustine with one, even the newer big boy battery one's wouldn't get through it. And if there is a lot of over growth and debris they aren't going to fair well. Even the cheapest gas powered mower will chop through tall grass, and if they conk out in the process, you usually haven't done any permanent damage.
But I really liked using a battery powered mower on a smaller, well kept yard. They are usually super light in comparison to gas mowers and so it makes for quick work. And they are much (much) more quiet. And depending on your fondness of two-stroke engines and their smoke, they don't smell as bad either.