Watched this movie recently and overall it was better than I expected. The general storyline and acting was quite good and the special effects - e.g., walking through solids - were actually pretty decent. Robert Lansing and Lee Meriwether were pretty good especially considering this was their first film. The other actors were OK.
However there were a number of shortcomings and unintentional funny moments: 1) The soundtrack was totally over the top. Very 'hip' beatnik jazz orchestration with bongos etc. This cartoonish music often played during some of the tenser moments of the film, effectively ruining the suspense. 2) The sibling rivalry subplot at the beginning of the movie was really unnecessary. It was probably added to heighten the drama and give the characters more depth, but in the end it was all rather pointless. 3) The funniest moment of the film occurred when 'Tony' was working on his fourth dimension theory at the restaurant table. When the camera showed his notebook, instead of mathematical calculations or a scientific diagram, it was simply a sketch of a block of metal with a pencil going through it. It was like something straight out of a Leslie Nielsen movie.
Overall, it is an OK movie and worth watching if you enjoy old sci-fi movies. It won't change your life, but you won't regret watching it either.