Haven't finished this game yet, but I keep laughing aloud and clapping with delight as I play with it, so I'mma give it five stars.
Others have noted how well-written it is. Being a writer myself, I marvel at the craftsmanship! Writing clear descriptions is a hard trick for some of us to pull off, but the writing in GBVB's practically invisible, a window into the world of the box. Add in the ingenious use of "U" and you get all the delights of physical puzzle-solving minus the annoying fiddly bits.
In short, this game is the philosophical opposite of Hard Puzzle.