Ive played the flash version of this game already, and I must say it is one of my favorites of all time.
This mobile version is very similar, keeping the games awesomeness. A few downsides:
1- the challanges are per game, which really doesnt add a replay value aiming a full challenge completition and making it really hard to achieve
2- some content is only avaiable on hard mode, making it a must if you want to explore the entire game, and some people may dislike higher difficulties as an obligation
3- theres a point in the game where you must choose between two units, making it impossible to recruit all units in a single game (I may be wrong on this)
4- very expensive exclusive characters as in app purchases.. The price is very incompatible, I think maybe $5 for all of them would be a fair value
Overall, really great game and a must to kill some time (and enemies), tho with some room for improvement