I hate IAP, I won't buy it, and the games that have it are irrevocably broken because of it. The model leads to bad games focussing on grinding. I have and will continue to pay decent prices for decent games (most recently, FTL, well worth the $10 on iPad, and Monument Valley for I think $5, which is too short, but tres awesome). Even these 'expensive' games are still chump change - if you're not prepared to pay even the price of a beer for a game that you'll get hours of entertainment from I'm not sure how you can expect people to write them for you. I'll happily keep chucking money at people like Capy and Vlambeer.
There was one exception - I bought the expansion pack for Scribblenauts Remix because I enjoyed the game and wanted more of it - but that was buying extra levels a year or so after the original game was released.
I really don't understand the idea of buying head starts, extra lives in infinite runners etc (I don't understand why anyone playing them would want them, because all it does it make the idea of a high score table utterly pointless, and that's all the game is).
Bought and loved PvZ. Haven't even opened PvZ2 because of IAP.
Also, I don't see the point of locked content at all. Just open it all up, let everyone try it. Why restrict it access to people who get to later levels? What purpose does it serve?