Something most people don't know either, you get 10 coins a day for each gym you have a Pokemon set up to defend (up to a maximum of 10 gyms) - giving you up to 100 free coins every day for just playing the game. And since you can't buy anything that you can't just get from a Pokestop either (and they are pretty common even in my suburban area, there's three on the ~15 minute walk to the convenience store down the street) there's little reason to spend money unless you are sitting in your house all day playing instead of actually walking around (which is by design).

Really, the biggest complaint I can see about the game from an IAP standpoint is rural areas get pretty hosed because Ingress (where all of the AR content came from) didn't have many portals in these areas, so there's a lack of gyms and pokestops to actually use for people living there. Hopefully Niantic makes some improvement here, because while Ingress was a game focused on urban exploration and it made sense for that title Pokemon is supposed to be a much more accessible franchise.