Thanks, it's actually more common than you'd think particularly in certain genres. For big examples with a large player bases you have GTA Online and the Destiny games. Fighting games also tend to run peer to peer both because there is (usually) only two people in a fight and because they are quite latency sensitive.

My own interest comes from many years of building multiplayer games professionally and wanting to make smaller scale multiplayer games for fun without needing to invest too much in infrastructure.