I built this as my first real stompbox (because of the breadboard layout, extremely useful!), and when I hooked it up, all I got was extreme oscillation. Oscillation is a kinda complicated thing to fully understand, but here we can simply refer to it as extreme feedback in the form of major hum and screech. At least it was for me. If you are having this problem, the first thing to check is ground.
Now I, being the newbie I was back then, assumed that you could just connect the ground of input and output grounds together and save a couple holes. This, as a matter of fact, turned out to be the problem. You MUST connect your grounds of input and output into the ground of the whole circuit.
Just figured I'd post it, even if it just helps 1 or 2 people