You can watch it online courtesy of The Cinemated Man:
Moria calls it "an extremely well made film" and says it "was one of the biggest science-fiction successes in the period before Star Wars". 1000 Misspent Hours gives it 3 stars. Roger Ebert says it "is much better than I expected it to be. It is quickly paced, completely entertaining, and its philosophical pretensions don't get in the way." Variety calls it "amazing". Salon.com says it is
a dreadful film, a compendium of clumsy dialogue, one-dimensional characters, risible plot turns and long silences broken by incomprehensible meaningful looks. But it has stayed with us because in conception, if not execution, the film had something to say.