Sky Force 2014 is a top down shoot'em up that is free to play but has in app purchases that range from $0.99 - $1.99 per item.
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Pro Has a persistent leveling mechanic
Upgrades to the ship carry over to next playthroughs.
Pro Looks and sounds good
The game has high quality graphics, beautiful aesthetics and a great soundtrack.
Con Convoluted payment model
Time spent on upgrades and replenishing available ships can be reduced by spending stars (in-game currency). Additionally, after 1000 stars have been spent a store opens that allows removing ads, getting more stars and halving upgrade and replenish times for money.