Very cool indeed. Illusions like these are always fascinating. With some of them, it seems like once you know how the illusion works, you ought to be able to make it cease to work, by somehow deconstructing it. But your brain just won't let you do that. You hold a ruler against the lines, for example, and say "See? Both lines are the same length!" and Brain says "Trouble me not with your irrelevancies. Ha! I laugh at your ruler."