First off, there are two reason as to why this anime is rated so low at all anime databases ( AP and MAL being the main ones ), one is that there are morons who have the following mindset "Fanservice/moe is the cancer killing industry, Japan should make more mature anime for mature people such as myself", I won't go further because this is a review so it doesn't matters. The other reason is that most of the anime fandom watches anime through streaming, while I do think that it's wrong, you're free to do what you want, but there is a major problem while doing so with Horizon: streaming sites tend to get the first subbed episode available, and a certain group named Hadena was normally the first to release Horizon episodes. If you know this group you'll know why it isn't wise to watch them, I've seen thousands of people complaining about not understanding anything about the show so they say it's a bad show and etc etc.
Now, on to the review itself
Horizon's plotline is a bit complicated at first, it's not even because it's Sci-fi, it's more because the background itself is weird. But it's this weirdness that makes Horizon unique, even if it probably has been done before - I don't know all anime/movies/books y'know? - , Horizon still manages to pick your overused cliches at anime and turn them into something completely new. This isn't only for plotline, it also applies to characters but I'll speak about that later. A very, very resumed summary of the background is: something bad happened in our planet, forcing mankind migrate to an alternative dimension, but due to a certain organization, bad things started happening there too, so mankind was forced to migrate back to the original dimension aswell. But now said dimension is almost completely uninhabitable, with the East being the only place in the Earth where living is possible. Though at least 90% of the first season revolves around Toori trying to confess his love, the show also strays from this and shows the problems all the factions face and the consequences of each action. It also goes without saying that his confession has a huge impact on the story, so I can't really say there was anything wrong with the focus on said confession.
It's Sunrise, what did you expected? The animation on this show is amazing, from the explosions and clouds to the smaller effects each spell used by the cast, even the moments where there is no action - see examples like Guin Saga - where there's supposedly a cut in the animation, this show still manages to keep it on a very good level, showing the buildings and quite a few details about the place where the characters are walking. Long story short: it's really well done.
Now, this is my personal favorite part of the show, even if it has a great plot, animation and characters, the sound factor is what's astonishing on this show, it fits perfectly each situation, delivering the mood needed for each scene, from the most heart-breaking moment to the most epic fight, the music only increases more the effect of each scene, making you either feel bad ( or cry, if you're emotionally attached to ficticional characters like me ) or feel like jumping from your seat and starting a brawl with the whole city.
Now, here is somewhere I sadly have to deduct a bit of points. The reason? The cast on Horizon is FUCKHUGE! Forgive the word but that's the only way to describe it, there are as many characters as on a war-centered series, and even if most of them are really great and very consistent, there isn't enough focus on the cast itself. Quotting a friend of mine, "if you are watching Horizon thinking there is a main character, you're doing it wrong", and he's almost totally right, there may be a focus on Toori/Horizon but it's minimal, the focus of the show is on the plot, leaving aside most characters and focusing on those who are immediately "useful" to the plot, making the cast not-so-developed. But this was never a character-centered series was it?
Overall, this show is great, one of my favorites on 2011, if you like political conflict and can pay attention to the show, make sure to watch this! Don't listen to whoever says this anime is bad, those are people with the mindset I said on the first part, while some may indeed dislike this show, it's a show that is ridiculously fun but still serious, mixing both aspects perfectly. Oh, and don't worry about the cliffhanger on episode 13, new season is coming!