Really liked the idea behind it and the upgrade system. Its simple yet very smart/effective, wish i came up with it. The graphics follows the same concept, really simple and minimal but somehow visually interesting.
I think the upgrade system is the best thing about this. It compliments the idea of loop very well and I think that it has so much potential to it.
My only complaint is difficulty, the first wave is really hard and long and you have no upgrades making it somewhat tedius. It improves a bit in the second/third wave depending on the upgrades.