Very strategy-rich game, has lots of potential. Units have their own skill/spell tree, different combat interactions like parrying counterattacks dodging riposte, many atrributes and active or passive effects you have to all take into account throughout battle. Lots more classes/units planned as far as I saw on their wiki. Recommend to turn-based/strategy fans. It is not pay-to-win, since you get matched based on your army level - have a stronger army, then you face stronger armies. Don't have many criticisms, its already pretty complex and its only in closed beta stage.
EDIT (02/06/14): Now that it is in open beta, they have changed a few things. Some positives, like more missions/training modes, balancing buffs/nerfs, unit customization such as color and naming.
However, there are a lot of negative changes they have made. Particularly in regards to making the grind for in-game currency much, much slower. They've made missions restore only after 24hrs, and reduced success rate. They allow higher-leveled units to be only purchasable by real money, other than having to level them up manually. Experience gain is slower, and they increased prices for units. All in all, it isn't pay to win, but its very much free-to-grind. Like they say, you pay real money to be able to advance easier without as much grind. But the grind is very slow in this game.
Will update review if they make any major changes to the gameplay/grind. Everything in-game is still unlockable through in-game currency, you can play entirely free. It is exactly how F2P models should be, but much slower grind. You have been warned, its still very fun :-) GL HF