It is a nice concept! The idea of upgrading your skills to improve the gameplay with it increase the chance of hooking the player for a long time. As feedback I can say that the sound track felt a little repetitive, also there's a noise on the background playing all the time, it makes the gameplay a little less adictive, improving it and putting some more SFX's would improve even more your gameplay. You made a nice job, the game has a good concept!