I haven't done the hardest difficulty as I think it's going to be too much for me but it's 10tons's best game since Crimsonland. It definitely keeps that back and forth of being a bit weak then being very powerful (swinging back if you're playing in the survival mode). Gets a bit dependent on random powerups later on though, which means that Hard difficulty might be a bit much.

If you get it, play through the levels once before tackling the survival mode to unlock all the weapons and perks.