I use one and it's surprisingly comfortable. The pad provides enough padding for the body, and I don't really need padding for my lower legs.

As others have said, it won't provide as much insulation as a full-length pad would, so if that's an issue I'd either use a full-length pad or improvise something. but for summer camping, it's a way to cut down on the amount of weight you're carrying. It may not seem like a lot, but it's based on the principle that you can only carry so much weight. If my sleeping pad weighs an ounce less, that's an ounce more I can carry of something else.