With a tagline like "What if Quake level mapping was a puzzle game?", there was no way this game could be bad.
Amazing concept, and with an extremely polished execution; everything from the textures, to the UI, to the movement, to the fact that the game's acronym is "BSP"... it all works to really drive the concept forward.
If I had to criticise something, I felt the controls for expanding and contracting blocks (brushes?) were a bit finnicky. I would've preferred those to be tied to a key, rather than physically moving back and forth, since it often didn't register.
Nevertheless, I love this game, and I would buy a full release in an instant. Great job!