First of all this game is fun, probably my favorite free to play game on the App Store.
However, the game has some very serious issues with it. I have not paid for any in game purchases because of the high likelihood of the app crashing and in-game resources being lost, and if you do decide to play I suggest you do the same.
Let me give an example: like most free to play games you are limited by how much you can play certain modes with energy. Unfortunately the moment you decide to play a dungeon that energy is spent and isn't coming back. This wouldn't be a big deal, but about one in eight times I play a dungeon the app crashes. That means your energy is gone, along with any drops you collected from the dungeon.
This game is definitely pay to win, but the pve elements are more core to the experience and are super fun, so I don't mind. If the crashes or the way energy was handled in the crashing situations were fixed I would definitely give it 5 stars and buy a few items to support the devs. I can't really do that in its current state, however.