The few first levels, like 3, are meddling ugly. Plank everywhere, ewwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww. 4 was cool tho, it had this confusing feeling when the walls, floor and ceiling looked the same, but i'd change the transparent texture to other, that metal texture looks really ugly when it's transparent. The box level was also really ugly, because of the walls and boxes. Dunno, atleast change the plank texture.
Oh, few last levels are cool