I purchased IAP for Battle Nations twice; once because I wanted to throw them a bone, and once because I wanted a couple higher-tier units. These days, I feel silly for it, because I hit the later-game "you won't pass this without IAP or near-infinite patience" wall, and felt suckered.
I bought some gems in Clash of Clans for the same reasons, but now they've become more generous with the premium currency for free in game.
Considering how many hours I played each of those games, I'm still into them for way, way less money-per-hour than any retail boxed game, even a used retail game.
