I don't write reviews but the sheer preposterousness of this drama compelled me to do it. The following contains spoilers, possibly major? Depends on your perspective.

Story:
The alien's power does not make sense at all and it almost drove me crazy. Do Min Joon is supposed to be an alien from whatever planet, right? Well no, it's more like he's Son Oh Gong. I am not sure what the writer was thinking because since when does being an alien mean you can stop time and teleport, not to mention use telekinesis? Do Min Joon should've been an eastern mythology/magic character if they wanted to add all of that because it didn't help the story later on. In eastern mythology drama's God's will or a curse or something may hinder the lead from using their powers and it makes sense. But in this.....it just is? Frustratingly, the alien dies if he kills a human, which is so convenient to prolong this drama's washed up story. It isn't explained why either, because it can't be explained if he's an alien! Why would some heavenly force kill him if he kills a human? What connection is that? How is a human different to an ant to him, since he's alien. What law is creating this problem? It's complete bull. It is like saying "you will die if you kill a fly using your bare hands", but who's going to kill you? Is your hand going to snitch to God or mother nature or science, that you killed a fly with your hands instead of a using a fly swatter or gloves?
Ok fine, he can't kill anyone because of this ridiculous law, but he can cripple or injure right? Nah, he'll just run around giving hollow threats and wait for the worst to happen! He could even lock the villain in an isolated chamber but nope. For this reason, it would've been smarter to make the villain an alien too. But I guess the writer didn't think at all.
To make matters even worse, this guy can go poof into fairy dust if he stays on Earth forever. How does that work? Again, this isn't a magic show, it's sci-fi. It would work if they said something like "Earth's atmosphere contains toxins and he's been inhaling them for 400 years so he'll die if he stays any longer", but no they just think it's ok to make vanishing a thing. Is this guy an Angel or something? Like I said, the writer doesn't know how to write aliens/sci-fi realistically and utterly fails at even creating a semi-realistic version. It's almost as if the Screenwriter did whatever pleased her the most, without meaningful thinking.

They spend episode 1-16 in the one-sided love/distance crush part. There's just no progress and when you think there finally is, they crush it instantly. It's a worse back and forth than A Korean Odyssey. The writer focuses so much on driving a wedge between them all the way along the journey, by episode 15 they're still haven't graced mutual flirting. It becomes a mental chore to watch them bicker like kids. So the romance in this is a real headache. There's no happy period of mutual love in between like usual. It's infighting, one sided love, mutual love but distanced, noble sacrifice and finally mutual love. The whole thing is just an annoying tease until episode 16+.

Cheon Song Yi is so pitiful, her character is completely useless and a total airhead. She's not annoying like others say, it's just sad that she isn't given anything meaningful to do. She can't even do simple things like take care of a sick person at first, the writer doesn't give her any self-respect.
Whereas Do Min Joon is stuck between being a closet p*dophile and an outright douche. He's so aggressive towards Song Yi, even when he's started liking her, he just shouts at her angrily.

The drama doesn't progress at all until about half way through, for a 21 episode drama it wasted a lot of time dragging things out. They could have explored Song Yi's celebrity struggle more, gave Min Joon something to do other than run to Song Yi's rescue and act elusive for 11 episodes or more aliens. There is nothing interesting to keep you going apart from the alien action scenes and a deep, resentful longing for the nonsense between the leads to stop and a relationship to develop. The villain goes about doing whatever he wants because of previously explained rubbish. And almost all the side characters are rivals or evil or selfish, whatever negative. The entire story is built on Song Yi not finding out about Min Joon being an alien, they try to hide it for an unnecessary amount of time to keep the show going. And why doesn't this amazing alien that can travel through space have any means of communication with home? These guys are so high tech that they can travel light years in UFOs. They must have advanced phones or help beacons. Furthermore, the ending is bittersweet so it doesn't even have that going for it as a positive...

Casting is fine, apart from yoo in na who just can't act evil enough. And the villains hairstyle needs some fixing.

The standards in 2013 must have been supremely low, because there's nothing special about this drama. Perhaps the amazing special effects and casting lead to this much of an overrating, which makes sense because the sfx looked great. But any modern fantasy drama can outdo the story within 3-4 episodes. This can never be a 10/10 with all the bad writing, plot holes, hollow characters and milked out drama. The pacing of this show had its brakes on 24/7, preventing meaningful advances. I wish the writer had got some advice/opinions from peers because clearly this isn't her best work. LOTBS and CLOY are much better in every department, even though they all stink near the end (seems she never learned how to get away from this one troll ending type...). Overall I do not recommend this show, it's an agonizing waste of 21 hours.