I downloaded and tested this because of your comment on the Block Swapper made by minecraft-pg5, found HERE. But, I didn't like your design as much as his. I like his better simply because of the piston extender. That's exactly what I was looking for, which was for the block (in my case, a workbench) to come OUT of the floor, and not be flush with the floor like in this one. Still, good work. This design is probably the fastest-switching one that I have seen so far.