Along with a few others, I too, must chime in with my 'thumbs up' opinion of this lost classic. I was fortunate to see nearly ALL of the horror/sci-fi 'Classics' in theaters during the 50's binge..when there was a new double feature nearly every other week.
This one, stands quite a bit above all the others. The creep factor is high and it has some truly haunting moments. The piano score just adds to the muted terror. Cheap? Yep. But I think that adds to the atmosphere.
Sure, I was a 'kid' when I saw all of these back then, but only a handful of these films were 'great'. 'Colossus' is hands-down one of the best of that era. If you watch it in the proper context, I'm sure you will agree.