I am super-pleased with this thing!
It's got a footprint of 6x5 blocks, and only uses one slime block, two pistons and two repeaters per level. Plus a tripwire and some redstone dust, of course. Works allllmost 100% of the time, and is easily resettable if you get stuck - just walk off the slime block you ended up on, and you should finish your journey in one piece.
I also love the down-punchinator that I showed in there. Two-tick delay between the inverted "trapdoor" piston and the downwards slime block gives a nice little boost to downward speed that's almost totally pointless! Just my style!
(In other news, I just made gfycats embeddable...)