Elite Dangerous

If you already have the main game, and are interested in Horizons, (and you should be because Horizons opens up engineering and a lot of other stuff), you should be aware that this DLC is going to be included in the main game during October 2020 at no extra cost.
Contrary to some other reviewers opinions, this is NOT a dick move by the devs. There's a new DLC coming out early in 2021. Some other games (Guild Wars 2 for example) add the DLC to the core game when new expansions come out.
For those complaining, you got to support the devs, and play the DLC content. However at this point Horizons has been out for 5 years now. In addition you're going to be getting a skin for every ship in the game which alone will cost you more than you paid for the expansion.
Some people these days appear to be so so entitled. And can't stand other players benefiting from a dev's generosity.

Essential if you like the base game. Infact, essential even if you only kind of like the base game.
The Horizons DLC adds the following
-The ability to customize your character (minor)
-The ability to be onboard other players ships (cool, but not major)
-The ability to land on some planets and moons (MAJOR)
-The ability to interact with the Engineers to further customize your ship (MAJOR)
It may also add other things... but I don't remember them.

FOR PEOPLE HAVING ISSUES GETTING HORIZONS WORKING ON STEAM.
You don't need to do any of the Frontier website stuff the launcher tells you to do! Another user has posted the correct operations to get this running (but it's just a response to a review, easy to miss), as this is what Frontier support told him to do, I'm just going to put it here, where it's easier to find:
"Launch the standard base version of the game and under the top right 'Options' tab on the launcher select 'Log out machine'. From here you should be able to select the 'Steam Login' option.
This will make sure that all purchased content through Steam is recognised and you are able to get into the game and play the expansion."
This got it working for me, hope it helps others. And yes, this is some some cryptic stuff, no idea how they expect us to figure this out...

This is just part 2 of the base game. Elite basically requires this or you will struggle to have any fun. I'll give it a thumbs up ONLY because this is not a pay to win game. They've done a good job of not forcing people to pay extra to win or a monthly subscription. We all struggle through the grind together for one flat fee (okay fine, two fees).

Elite Dangerous (ED) is probably a boring game and for me, thats perfect. I have the Saitex X52 HOTAS, noise cancelling Kingston Hyper head phones, and a set of Oculus Rifts. ED is an immersive experience to behold. I put on a good iTunes station, crack a beer, place the Rift on my head, the Hypers on my ears, and I'm in a space euphoria. I spend my time exploring the galaxy beyond the bubble of game play. It's boring and it's beautiful. I'm at 293 hours of game play. I love this game.

This is the way to do expansions.
The content featured in Horizons is seperated in such a way that players that don't have Horizons can still interact with players that do: they just won't be able to follow Horizons players down to planet surfaces and use some of the things they can. It doesn't split the game entirely, which is the fate of some games that attempt expansions like this.
The content itself is worthwhile, as well. Landing on airless planets and moons is done excellently (it really is seamless). The planets are completely full-scale, and from the rocky planets we can land on now, can be up to 7x Earth's size. Gravity can also drastically alter the performance of your ship and buggy, which is incredibly cool. It really does make a difference if your buggy is stuck to the ground like a magnet or can fly freely for a long time before falling back down. One of the coolest things is that *really* small planets are not perfect spheres, and are more "potato-shaped," like big asteroids. This is both true to real life and adds really cool variety to the planets.
Of course, there needs to be stuff you can do on the planets, and luckily, there is. There are stations like the starports you dock at in the base game, hostile bases to take over, materials to gather, and introduced in 2.1, engineer bases. These are places you can land at in order to craft material you can collect into weapon and module upgrades. The entire system is really cool and adds a lot of customizability to your loadouts. There are also random landmarks you can find on surfaces, like crashed ship wrecks. One interesting storyline unfolding is the discovery of alien "barnacles" on planet surfaces, which hints at aliens being added into the game.
Horizons isn't just planetary landings, though. It's an entire season of expansions: planetary landings was just the first expansion in the season. 2.1 brought Engineers and crafting, and upcoming updates are adding more things. 2.2 is bringing ship-launched fighters (the ability to use large ships as hangers for smaller ships to launch out of), the ability to hire NPC crew, as well as several other quality of life improvements. Many of these things are for all players of Elite Dangerous, not just Horizons players.
Farther in the future, 2.3 is bringing multi-crew, which will allow real players to appear in your ship as crew. The update also brings commander creator, the ability to customize your player creator. This should give us our first taste of the next big speculated step of Elite Dangerous, being able to walk around outside of your ship (probably the headline feature of the next big season of expansions). 2.4 hasn't been revealed yet, but it's been speculated to be aliens.
One of the most common criticisms of this is that it's too expensive and that Frontier lied about features in the expansions that were supposed to be in the base game. As for the price, I think it's fair now: two of its major expansions have been released, so $30 is fair. $45 for the planetary landings when it was first released was definitely pricey, but they listened and dropped the price soon after. It's definitely worth it if you're an Elite Dangerous player. If you're not one already, you can get the base game and Horizons together for $60.
And I cannot emphasize this enough: THERE WERE ALWAYS GOING TO BE PAID EXPANSIONS, AND WE ALWAYS KNEW WHAT WOULD BE IN THEM. THEY DID NOT LIE. Go to the Kickstarter campaign and Ctrl-F "expansions." Go watch the Kickstarter video they uploaded in 2012 called "Elite: Dangerous Development Plan." Go through their newsletters: it's mentioned in at least five or six times before Horizons was even announced (newsletter #32 most notably). Look up the dev Q & A "to infinity, and beyond!" Those are just the ones on the top of my head. You'll be hard-pressed to find any time they mentioned planetary landings or any other big future feature without mentioning that they'll be paid.
Overall, Horizons is a great addition to Elite Dangerous. It was rolled out really well, doesn't split the community, and adds content well. It just shows Frontier's excellent development plan coming to fruition. They're slowly but steadily building a game that will be legendary.

Pros: Includes everything that should have been included in the base game
Cons: Includes everything that should have been included in the base game