It's fun, it's different, and the pacing is very good. I found two problems though. First, once my digging was going really fast, the game lagged so much that my progress essentially stopped. And second, it seems like there's either no end - I got up to 180 million feet, and the Earth's diameter is only about 40 million. So, for me, the game ended in a laggy anticlimax. This took something like two hours.