I'd give it a B-. We had a couple of goes, but couldn't quite get the snake effect to work as we wanted, but we still enjoyed doing it. If it was anything, it was closer to a Black Octopus than a Snake.
I'm just going to link to this page. I didn't do much chemistry at school so I don't really understand the reaction myself. Suffice to say that the bicarb and the sugar release CO2 gas which puffs up the ashy residue into a snake.