The look and feel of the game are really well made. One question I was having during my playthrough was if there are other elements other than the boxes which can be used on the black hole-portal machine. For example, I was trying to see if I can somehow use the black hole to absorb the bullets in the final level and somehow use it against the shooting machines: does it only affect the boxes nearby?
The only minor glitches I encountered were I couldn't pick up the portal gun on my first playthrough and the gun usage text went away before I could read it, but other than that (which worked fine on re-run) it was a really well-made and polished experience :)