I haven't tried one, but I'd really like to. They look really neat! And it'd be nice to have more desk space. On the other hand, it'd be weird to not have my fingers hitting anything when I type. Might throw me off for a while.
They used to be sold at thinkgeek.com but no longer. I had a buddy that had one about 5-6 years ago. He used it with his Palm Pilot. I remember it being ok but not great. You could also turn on a clicking sound if you wanted. It still felt like typing on a table. It not reminds be of typing on a flat smooth surface (like an iPad or such) except the keys would not project "through" your hands. Since it projects above that point..your hands would have the red outlines on the back of them.
All in all..at the time it was 250-300 when he got it. I don't think it was anywhere near that. I personally had a roll up keyboard for my Palm at the time.
Cool indeed. But I can't imagine typing on a hard surface. I might as well put a quilt on the table before I use it--if I ever get one, that is. Might be useful for other purposes though, I just don't think it would suit my need.