I fell in love with Master of Orion II when it came out first (1996) and since then I look for a worthy successor. Played several 4X games and they had their pros and cons. Endless Space is one of the better 4X games and absolutely worth buying, in my opinion. Wouldn't have played 300+ hours else, heh. I am reviewing it including Disharmony expansion.
Galaxy: Endless Space offers some variety of galaxy shapes and sizes, some favoring defensive players, some favoring offensive ones. Additional finetuning of the galaxy is possible. I miss the opportunity of creating a REALLY huge galaxy, but maximum size with just one opponent works ok as a sandbox mode. During exploring you will find nice bonuses, a few pirates, wormholes (basically dividing the galaxy until you can travel through them) and finally different wonders, so exploring is actually fun.
Background story: You learn about the Endless (usual Elder civilization which went extinct) during playing. The game lacks lore summary articles, but there are traces of this ancient civilization everywhere (galaxy wonders, technologies, factions, leaders), which adds to immersion.
Factions: The games comes with 10 to 12 moderately unique prebuilt factions (depending on whether you get Harmony and Vaulters). Each gets specific traits, innate bonuses, ship bonuses and a few special technologies to encourage certain victory types and hence playstyles. Prebuilt factions are different enough to motivate you to try them all. Additionally, you can create your own faction by modifying a prebuilt one. Meaning you can replace their traits completely, but you have stick with their innate bonuses, ship bonuses and special technologies. It has still a high degree of freedom, I enjoy building my own faction a lot.
AI: Computer opponents (artificial intelligences respective AIs) offer a solid challenge. Especially the 'evil' factions tend to attack you early on, so you cannot simply expand all over the galaxy or rush through the technology trees. Later most AI will attack you when they run out of colonization space - unless they are busy with other opponents. But they also attack each other for same reasons and their reasoning is mostly transparent (+x for long period of peace, -y for common borders etc.). At higher difficulty ratings they are more unfriendly (but not hateful) and get the usual strong production bonuses to offer more of a challenge.
Leader: You can hire a few people for boosts to your empire or your fleets. They make a real difference and usually have a nice background story. It takes some time to learn about their feat trees, but once you became familar with it, your leader can turn a fleet into a nearly unbeatable force. Which you will need to fend off the many enemy ships at higher difficulty ratings... Using leaders to improve your empire helps a lot at the beginning but loses importance as it grows.
Production: Production is done on system level mostly, saving you some time compared to games which do it on planet level. So most buildings affect all colonized planets, ships are produced by the entire system etc.. However, you can specialize your planets (usually: food, industry, research, money), explore their moons and terraform the planets. Terraforming is an interesting feature here because depending on faction and situation you can be better off with unfriendly environment. Since production shouldn't be automated, you will spend a lot of time with tweaking your systems. But that's true for many of these games...
Technology: There are basically four technology trees and they sometimes offer different paths to specific technologies. It honestly needs quite a while to get used to them, but it offers interesting decisions to make. Technology names are often NOT related to their benefits, so at the beginning you will have to check their descriptions often. Other games did that better. Well, on the plus side you have many interesting concepts in the tech description texts - some scifi fans will like them.
Diplomacy: The negotiation with aliens is a solid feature. At the beginning you are at "cold war", meaning you can invade other factions' outposts but not colonies - a bit confusing first. Of course there are war and peace. Peace can be upgraded a bit by two more treaties (open borders, cooperation) and there are alliances. Alliances are usually a short-lived thing with AI empires, but a few traits and technologies reward you for allying. Upgrading from war to cease fire to peace to additional treaties / alliance goes slowly since there is a hardcoded turn amount between them. For instance you have to be at war with your opponent for 10 rounds, then both sides can offer cease fire. At the beginning of the game this makes some sense (why ally instantly with some empire you barely know?), later it becomes kind of a burden. AI empires sometimes ignore the turn amount, speeding up the process. Beside this you can trade dust (basically money), technologies, systems and special resources. At the end of a long war you can even make an AI empire give you all its colonies.
Covert operations: There is no secret service in any empire which is a bit of a letdown. Leaders can steal some dust (money) and technology points from other empires, but that's it.
Ship design: This game gives you rather much freedom how to design your spaceships. The weapon choices are a bit dull: Level 1, 2 and 3 versions of mass drivers, beam weapons and missiles, plus some fighters and bombers. You can defend your ships with thick armor, defense systems or simply taking out the enemy ships fast enough. Finally some special systems make your ships truly unique: Better engines for raids, better scanners to avoid getting surprised, siege for slow invasions, marines for fast invasions (they cost population!), multiple colony pods for quick expansion or multiple layers of armor to accept and shrug off enemy hits.
Battles: Endless Space has a somewhat simplified battle system. You cannot control single ships or squadrons, instead you set the overall strategy by picking a card and probably refine it by formation and general targetting tactics. Leaders add a bit of complexity with their special cards. 'Manual combat' just means you can watch the battle (which looks nice) and change cards every combat phase. Of course this card system means you lose some control (compared with other games), but it also reduces micromanagement and makes the AI more competitive in battles. Some people hate it, some love it, I'd say it's ok.
Bugs: Endless Space crashes ocassionally, let's say every other game. In every case reloading the latest savegame fixed the issue. According to the error messages it's mostly about AI diplomacy - playing with less opponents and in smaller galaxies might help. Given the fun the game provides, I accept the rare crashes though.
If you like complex 4X games, you will (very likely) enjoy Endless Space and its addon Disharmony.