Googlevideo has it online:
"What is out there?"
Moria gives it 4 stars, likens it to Stalker (one of my favorites) and says,
it is welcome to find a work that derives its impact from carefully etched characters and the playoff of their tensions. Cube is a beautifully constructed puzzlebox of a film – a work of strikingly original intelligence.
Slant Magazine has a review that says,
The struggle to discern the cube's purpose increasingly takes on prominence in the frazzled crew's search for answers, but Natali's film is infinitely more competent at creating a clever situation then positing any sort of semi-logical explanation.
Variety closes by saying that "the tech aspects are so accomplished and intriguing, with Mark Korven's minimalist music repping another bonus, that pic is bound to attract a cult among home-box prisoners." The New York Times says it "proves surprisingly gripping, in the best ''Twilight Zone'' tradition."