I played for free for a long time until I ended up with so many resources to manage that I couldn't do it well anymore. I had to pony up the $6/mn to get the advanced features.
The problem with these small transactions is people don't see how much it's costing them. If they priced the game at $20 no one would buy it, but they'll be willing to spend $100 if it's spread out of 100 $1 purchases.
One good thing there is that they explicitly limit how many items you can use, and you can earn the items in game. Still, anyone trying to rush the start of the game probably wants to use them. Sometimes I fantasize about blowing forty bucks on that. I think I've been skinner boxed.
i'm seeing similarities with the clash of clans model to the idea of putting a frog(player) in a pot of water(game) and slowly turning up the temperature(tiny payments), until its boiling and they don't notice the total sum of the payments along the way...