The idea is to make it looks like I'm sitting in a chair riding atop a steam-powered mech that walks on two robotic legs. In reality my seated legs are false, and my real legs power the robot's legs. This is basically a variant on the age-old circus-clown costume where someone looks like they're riding a horse, and is also inspired by Ben Hallert's APU costume and the paintball mech costume called Steel Dawn.
While not fast enough to keep up with 6-year-old trick-or-treaters as they went from house to house, I was still able to walk down the street and show off to passers by. The most common reaction was along the lines of "Wow! That's the coolest costume I've ever seen -- what the heck are you?!? I also got little kids (and some older kids, who really should know better) asking me how the thing was powered, several adults admitting they couldn't figure out how the thing worked, and at least one little girl bursting into tears as she saw me ambling towards her. All in all, I'd say it was a big success :).