The usual issue with old SF movies is, well, that they're old, and often don't age well. Not really with this one.

First it has an interesting subject that could still be done today. I mean, discovering another planet that we could never see before because it's on the same orbit on the opposite side of the sun was probably as stupid an idea as it is today, like, how could it have gone unnoticed all this time ?

What's really original is the whole visual style of the movie. The decor, whether insides or buildings, the clothes, the furnitures, it's all pretty unique, and because of that it doesn't have the old sf feeling, it does not look like what was the futuristic norm of the time.

It's pretty long until the space mission but interesting, for instance the astronaut training scenes. It's too bad that the whole concept ([spoiler]that the planet is a reversed copy of our Earth[/spoiler]) is under exploited (I'll keep it as spoiler but it's pretty much spoiled by the movie tagline and when I saw it by the guy introducing the movie). It's more turned into a kind of mystery thing with a pretty weak reveal and a conclusion that seems to indicate that nobody ever tried to go to the planet again.

It never tries to work with the implications. For instance [spoiler]it's an exact copy, up to the point where they sent astronauts to the other side at the same time[/spoiler]! There's a missed philosophical theme on free will. But also could there be some other difference ? Could they experiment on some stuff ? There's a whole scientific theme missed here. Would the arrival on the planet starts to change that ? There could also have been some kind of diplomatic treatment ([spoiler]after all he's not theirs, he could be a spy, he could represent a danger[/spoiler]). Etc.

Anyway, even if it's 50 years old, it's still highly watchable.