I know someone who's used them. He commutes on them only. They are solid rubber with air bubbles impregnated. Really hard to get on the rims - the company gives you a tool with them or you'd never get them on. Sheldon Brown has a write-up on how these type tires really defeat the purpose of a pneumatic tire and are very hard on rims. Probably hard on your butt too. The synopsis is that when a tire with an inner tube hits a pothole, you are compressing air in the entire tube volume as one area of the tire compresses. With air-free tires, you compress some microbubbles of air on only that area of the tire. There just isn't much capacity for shock-absorption.