Really liked it overall! I was specially a fan of how strategic combat feels for an action game. Keeping check of how many hits it takes you to kill an enemy and the speed of your attacks is very important to success, and since enemy stats don't change through the game, it really feels rewarding to keep a close look on it.
I also really liked the walker enemy AI. It was smart enough to be threatening, but also relentless enough that you can lure them into traps with the disappearing blocks or waiting in the proper spot.
Regarding the upgrades system, I was suspicious of it, since it looked like it could lead you to "lose-more" situations, but it really combined well with the weapon system. I found myself going for normal rewards for levels I found myself unprepared for more than I expected.
It's getting late, so I better get some sleep, but I'll try to give it another go, I really want to kill at least one group of golden skeletons, they look great.