I finished the game in an hour just then, it is a great game based on a great concept. At some moments in the game I would get stuck and then once I figure it out the answer seems so obvious. There were a couple of bug that I found throughout the game, 1. the cube would bug out when placed on the edge of a portal, 2. you can't place a portal below you sometimes when on the side of a wall in 0g after getting there with a sprint jump and 3. (not sure if its a bug) you move REALLY slow along the roof when you don't do a sprint jump on the 0g world. The game was challenging and frustrating once you got the the main world but definitly worth playing.
some other minor things you could change would be the tutorial only lasting a small amount of time (I only realised you could use R in the final levels)
portals can take a second to open
sensitivity was a bit low (although I was on a touch pad)