All things told, this was a really good game. A nice blend of challenge and killing all humans.
Difficulty progression is solid without making it feel like you need to grind in order to beat any single level; by the time you've gotten to Level 15, you should have just about every worthwhile upgrade anyways. Naturally, you're going to want to go for damage immediately, but otherwise "Shield Recharge" and "Shield Capacity" might be your most important upgrades, followed closely by "Ray Capacity".
The only thing that I might say I'm indifferent to are the options in the provided "cool menu" as they basically take any challenge out of the game, and seeing that you need all fifteen achievements to unlock it anyways, the reward seems pretty middling. Even without that, though, the game is really solidly built.