Elite Dangerous

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.

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

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...

Disclaimer
Read my previous review of the basegame here, as this will only add upon all I've said there.
What's New
This DLC is essentially one massive expansion, adding more depth to many previously seen aspects of the basegame. Most notably the following:
Landable non-atmospheric planets (planetary city-bases, driving around in a small reconnaissance vehicle)
Many surface things to explore, ruins of ancient civilizations, aliens, interstellar life forms, geysers, etc
Engineers, certain NPCs that will allow you to modify your ship's modules to your liking (this is a must have for PvP players)
Aliens (thargoids) which are unique enemies and represent 'end-game' combat opportunities.
Many new ships
Larger ships can now have fighter hangers that launch smaller nimbler fighters that are either player crewed or you can hire an NPC to pilot them.
Multicrew (have other players join your ship as short-term crew for manning turrets or smaller fighters from big ships)
Customizable player commander looks (not sure why this isn't included in the basegame though)
New unique unlockable weapons and modules
Landable Planets
This includes only planets that do not have atmospheres, excluding the Moon, so if you're planning on landing on the Earth, Mars, or other planets and exoplanets that have atmospheres, you're out of luck. This might change in the future if they finally implement procedural vegetation and cities in the next expansion, but not until then. Luckily, only around half of the planets aren't landable, and the other half are.
There are many unique types of landable planets, featuring different colors, terrain, canyons, tall mountains, etc. But as with every procedural game, it can't make every planet feel unique, and after a while they all kind of feel the same more or less. Some have unique features like patches of interstellar lifeforms (mainly in the form of plantlike life), geysers, abandoned settlements and alien ruins and bases.
Engineers
These are special NPCs that have their own unique bases that offer the player to modify their modules (mainly improving them) in exchange for materials that the player can gather from around the galaxy. Need more firepower, engineer your weapons for higher damage but increased power consumption. Want more ship integrity, engineer your hull for greater protection but increased mass. Require more power, engineer your powerplant for more capacity but increased heat output. There's engineering for pretty much everything on your ship. Even if they have certain drawbacks, they are very efficient and definitely an overall improvement of your ship. If you're planning on doing serious PvP, engineering is a must have.
The bad things that come with engineering is powercreep and the grind required.
There are upper limits on engineering of course, but an engineered ship can be twice as good as a non engineered one. On the other hand, some engineers require quite a grind for them to offer their services, and even then, the act of engineering requires special materials that can't be bought with credits, they have to be gathered by you. Some rare ones are particularly difficult to find, but luckily there are material traders that will allow you to exchange materials for other kinds of materials (at a certain loss, but it is conventient).
Thargoids
Bored of destroying human ships, pirates and fighting in boring human wars? Looking for an endgame combat challenge? This is then what you want. These aliens are returning and are a direct threat to humanity, refusing any communication and destoying anything in their path. While there is a certain story and narrative behind them, they are very much hostile only, providing a decent challenge even for the best pilots, many of which require groups of players fighting together to take them down. Many of them require special strategies, engineering and weapons for taking out. There are quite a few things about them, and how you can fight them, how they affect human starports and the effects of their presence within the galaxy that I won't cover here. There are dedicated player groups in-game whose primary role is to fight them back.
Multicrew
Probably one of the least popular feature, mainly because it is used pretty much only for combat. This is a feature that lets your friends (or strangers if you desire so) help you crew turrets on your ship (if your ship is capable) and man a smaller fighter that can be launched from your ship (again, if it can have one to begin with). It's a decent idea but it isn't really well done, and lacks more depth. It's only used for short sessions, you can't walk around your ship and explore, it's just having someone in the other seat controlling either turrets of deploying and controlling the fighter.
My main issue with this is the poor netcode around it, even if you haven't had issues playing with friends regularly, you will probably experience it here, especially when trying to jump systems. It's dodgy at best, and broken at the worst. It can work decently if you prepare beforehand, but traveling while in Multicrew is going to be a pain unless you both have a very relaxed router and firewall. Read the 'issues with multiplayer' section of my previous review of the basegame for more info.
Upcoming 2018 Updates and 'Beyond'
With the release of Horizons, the next series of updates and content (called Beyond) has been made free for all owners of this DLC, many of these changes and additions impact Horizons content directly, so if you own the basegame but not Horizons, you might not be able to directly interact with all additions (for instance, new stuff to discover is mostly based on planets, so players that cannot access planet surfaces won't have access to the new content, while other changes, for instance, changes of crime and punishment, will be reflected upon the basegame). 'Beyond' is releasing in 4 chapters, with the last one being scheduled for december 2018, after which a new paid expansion will be announced (no details on this as of writing this review).
Final Thoughts
If you enjoyed the basegame, I would highly recommend buying this. It adds a lot of new content and things to see and do. The game just doesn't feel complete without it. Even if you're well into the hundred of hours of basegame playtime, it's never too late to pick this up. It often goes on sale for 50% off, so be sure to check frequently. I'd still recommend it even at full price, though.

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.

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.