It's a 1950s SF film. It's supposed to be one of the better ones, but even so it still has that era's naive storytelling and simple-minded sense of "gee whiz" that makes it a slight chore to sit through. I can understand why it was used in Mystery Science Theater 3000: The Movie.
What's good about it according to the contemporary critics are the special effects (agreed), well-written script (ahahaha) and eye-popping colour (eh?). Personally I was disappointed that it wasn't in black-and-white, but to be fair they cope well with that handicap. The film doesn't look cheesy. There's enough 1950s retro in the designs to give the film character, but not so much that it's silly. I was impressed. They've put enough into the production values that it looks like a good SF film that happens to be set in this era, instead of the usual cheap nonsense. There's an alien monster, for instance, that looks hysterically funny in photos and yet in the actual film looks rather good.
Furthermore the special effects are, for the time, groundbreaking. I even like the bit where the flying saucer dodges a bath sponge, although I must admit I was amused by the people with condoms on their heads.
As for the script, it's based on a three-part serial by Raymond F. Jones from Thrilling Wonder Stories. What's more, it has good points. Jeff Morrow's sympathetic alien, Exeter, is interesting, being a compassionate man forced to follow hard-hearted orders for the good of his species and as a result ends up committing treason. The movie never calls it that, but that's what it is. He gives the finale emotional weight. Morrow's character is clearly the best thing in the film and he deserved even more dramatic focus, although unfortunately the film never stops seeing the universe through Earthling eyes.
Apart from him, though, it's as half-baked as usual. Oh, I like the ideas. The Metalunan dilemma is intriguing, Morrow gets some good material and you could reasonably call the story thoughtful and intelligent. There's great potential here. Unfortunately though it's being forced through that 1950s SF wide-eyed story structure that's steamrolling its characters under the plot. Normal stories have heroes. They'll want to achieve something, but there are obstacles in their way. This kind of film though tends to have passengers rather than protagonists, for whom the plot is something to be stared at from a distance. That's an exaggeration here, but not by much. Rex Reason is reasonably active while we're still on Earth, but once in space he's a bystander. This film doesn't have enough dramatic meat for even an episode of an SF TV series, although you could certainly create one by building on its ideas.
Besides, it's ludicrous. You'd be mad to expect anything else, but even so. Here are a few of the film's goofs:
1. A spaceship that runs on nuclear power, built by a culture dependent on uranium, can crash and explode on Earth with no adverse effect on the environment.
2. A neutrino ray blasts a hole in a lead slab.
3. A civilisation with a critical energy supply problem has chosen to maintain a force shield around the entire planet, strong enough to destroy flaming space missiles, instead of just putting up a grid of the neutrino ray guns that their spaceships use for exactly that purpose.
4. If a man has betrayed his people and abandoned his civilisation to save you, the correct response to his decision to die in a pointless self-sacrifice is "oh, okay then."
5. The cutting edge of 1955 atomic science involves trying to create free energy by turning lead into uranium. In other words, alchemy.
There's some good stuff for the humans in the first half, though. Rex Reason gets suckered into a seriously dodgy project, simply because Morrow dangled alien technology before him and then asked nicely. Faith Domergue gets to be nervous and weird. There's a fun mix of truths and half-truths in what Morrow says to his captive scientists, although I don't see why his assistant's such a psychopath. The Metalunans end up blasting away at their captive humans as if they think they're in a Sergio Leone movie.
Oh, and after posting this review, I got an interesting comment from the esteemed Mr Simon Bucher-Jones. He says: "I've read R F Jones's "The Peace Engineers" stories. The film's a very good adaption of the first 2 (basically up to discovering the contacters are aliens, not a benevolent cabal of super-earth scientists) coupled with a rapid B-movie ending shoved on when they suddenly run out of film and don't have time to adapt the crucial end of the series - which incidentally is basically "Destiny of the Daleks" [two highly computerised empires, need irrational tactics to end stalemate]." Thinking about this, I realised that the storytelling in those first two acts is more solid than I'd been imagining. They're not fast-paced, but there's a lot of good in them and crucially they have a main character who's the focus of the story.
I haven't watched the Mystery Science Theatre version of This Island Earth, or indeed any of their episodes either. I believe they usually mocked worse films, but you can't say this wasn't offering them lots to work with. Do I like this film? Not really. It made me sleepy. However it's clearly ambitious, with a huge scale and lots of special effects that even today are still beautiful and effective. It's got an iconic alien, albeit one that's not involved much because they shoehorned it in because they wanted to have a monster somewhere. It's got morally complicated aliens, who are dodgy people doing fascist dictator things... but in the face of a fate so appalling that you almost give them a free pass anyway. Then there's Jeff Morrow, who's playing a surprisingly complex character even by proper standards.
It's just, y'know... the 1950s.