Very cool! I've also kept track of all the streets I've biked or walked down in both Chicago and San Francisco, although I didn't set about it as rigorously as the author, nor did I ever complete them. But it is a fun exercise in motivating you to get out there and to see parts of the city you might otherwise miss.

The difficulty is finding new routes all the time; at first, it's pretty easy, but then you start needing to go further and further out of your way to hit new streets, and the directions get more and more complicated. I have often thought about making an app to do help you with this, but I don't know much about mapping data and how to best store and calculate routes.