Journey to the far side of the sun (1969)

I watched this film last night for the first time in a long while. I remember it as a classic but watching it again I have to admit there were some parts of it that were a little disappointing now, probably down to the fact of the age of the film and the reliance on models for the special effects. The film is known as Doppelganger in America I think. If you like the live action Gerry Anderson TV series  UFO you will enjoy this film, there are members of the cast who are in both.
I think the main idea of the film is a good one but really I'd have to question whether it is a strong enough idea.