I got one to go with my Tantus Boss. It has the half balls, and was a complete FAILURE. For starters, the suction cup itself does not stay put. I agree, the floor will work much better than the wall. On the wall it just slides down - very annoying. Plus, my toy just came right off of it. It really doesn't seem like a very suction-y suction cup on either side.
Something I have tried that works is using a harness (like a thigh harness, for example) and tying it around a pillow. I've even used a really long length of sturdy ribbon to do that, and it works GREAT. Of course, that wouldn't be for shower play, but it still works for hands free fun.