Originally Posted by DaedalusAlpha
The upside down half blocks are interesting as they essentially mean you'll be able to build full height blocks at half the height offset of normal blocks (if you combine a half block and an upside down half block).
Yes, if each one is in a different full block space. This will probably be the only way you can combine two different halfblocks of different materials. It won't work if you try the same trick in the same full block space. And you probably won't be able to put things like redstone dust under an upsidedown half block.