So I played this game a bit. Installation was completely automatic (all credit goes to Steam - the game will be Steamworks based, including multiplayer managed by Steamworks, MP isn't in the alpha for now though).
Here is a video of me playing first 15 minutes of a new game (don't forget to switch to 1080p):
I don't have audio recording equipment, so the commentary is only textual.
I may post more videos when I get further into the game.
What is Endless Space?
ES is a turn based strategy, which puts it much closer to Civilization than to SoaSE. Just like in SoaSE there is no campaign (yet?).
Hardware requirements
The game seems to have much higher hardware requirements than SoaSE. For indie games, I'm used to crank up the graphical settings to maximum. I had to reduce the fidelity setting (there is only one + resolution and fullscreen mode) to "good", which is the medium value, to get nice framerate of about 30 on galaxy map.
My machine is Dell XPS L502X laptop with GeForce 540M, Core i7 Sandy Bridge, 16GB RAM and OCZ Vertex 3 SSD drive. I'm playing the game in 1920x1080 resolution.
The game will occupy about 1.6 GB on your hard drive.
User interface and graphics
The UI is jaw-droppingly beautiful, perhaps the most beautiful strategy game UI I have seen in years, the only game which comes close is Civilization V with its secession-inspired UI.
There are no 2000s-like pseudo-3D interface elements. Whole HUD is composed of flat-gray rectangles of various shades with simple symbolic icons. These icons are very elegant and readable as result of that. The game consistently uses clean readable sans-serif font (instead of awful futuristic fonts used by many sci-fi games).
The HUD itself is very unobtrusive, thanks to its simplicity and clever use of transparency. This makes the main content of the game window stand out - the galaxy in the galaxy view is always colourful and beautifully detailed (it may sometimes make the stars with planetary systems hard to see though). Planets in star system view smoothly float to and fro as you mouse over them to get more details.
Pretty much everything in the game has a tooltip, which details additional information about the object. This prevents the interface from being cluttered by hiding information that don't necessarily have to be presented at all times.
Much like in Civilization V, when something important happens, a stack of icons is shown in lower left corner of the screen instead of showing a modal dialog. This means you can decide what to do about that event (tech researched, production finished,...) whenever you like.
The interface also feels very snappy because transitions between screens are eased with subtle animations. I also love the blur effect which comes into play when there is a modal dialog window in the foreground.
I also have to mention few things I didn't like (which will be hopefully fixed in future releases). Currently, mouse can't be used to scroll the screen - this applies to both galaxy map and the tech tree. Galaxy map can be scrolled with arrow keys, but even that doesn't work in tech tree. - you have to constantly zoom in and out. Another thing that bugs me is slight inconsistency in graphical style - as I said, pretty much whole HUD is composed of flat shaded areas...with exception of end turn button and the event icons, which have a glassy look. Why?
Races
5 out of 8 races are so far available. There are normal humans (which are surprisingly considered to be evil), science race, proud warrior race, "planet eater race" and a weird race composed entirely of clones of one guy.
The races have various bonuses, maluses and racial technologies.
Galaxy, planets and colonization
The galaxy can be randomly generated according to several map scripts. So far I tried only "Spiral-2" which is 2-armed spiral galaxy. The stars in that galaxy indeed follow the shape of that galaxy, which results in rather "snaky" map (the two arms are connected by the galaxy core). The maps have many additional parameters, such as size, density, resource richness, planet type distribution etc.
The galaxy is composed of stars with their planetary systems. These stars are familiarly connected with "strings" (= phase lanes). To colonize a planetary system, you have to build a colony ship (you get first one for free, in addition to your starting colonized planet), which costs you one population on the planet where it is built in addition to its regular production costs. You then have to move this ship to the destination star and hit Colonize. Then you choose which planet in the system you want to colonize and voila, the planet is yours. Colonization of additional planets in already colonized system doesn't need colony ships, you can colonize them for production cost from the planet you already own.
Production and population work exactly like in Civilization - the planets have their food, production, gold and research stats, which can be further improved by various means.
There is just one production queue for each planetary system (which is shared for ships and buildings and other buildable projects). Some buildings and improvements affect entire system, other affect only a single planet.
There are many more planet types than in SoaSE - there is pretty much any planetary archetype you can think. The planets also differ in size - bigger or smaller size gives the planet various bonuses and maluses. The same applies for anomalies (additional special abilities which the planets can have). You need a specific tech to colonize the more hostile planet types.
There is much depth in this area which I haven't explored yet.
Research and Technologies
Research also works very similar to Civilization, except there are four trees instead of one - military, diplomatic, economic and support. Each race has some specific techs in the tree (those have orange icons instead of blue).
As far as I can tell (and this applies to pretty much everything in the game), the authors did their homework, which results in believable, but still very imaginative (and sometimes slightly humorous) game lore.
Ships
When a ship is produced, it goes into a "hangar", which is kind-of system-specific area for newborn ships (surprisingly, you can't see ships in this are from the galaxy map). Form here, you can assign the ships to fleets. Fleets is a group which moves and enters battle together. Fleets can be merged and split as you wish.
The game has a nice ship designer. As you complete research, additional hulls and ship parts become available. You can then design your own ships from these hulls and parts.
Combat
The combat happens separately from the main game. Once the two fleets engage, you can choose whether you want auto-resolve (self explanatory) or manual battle.
Manual battles are composed of three phases: long-range (missiles are used), medium range (beam weapons are used) and melee (kinetic weapons are used).
You can't actually control the ships as that battle, all you can do is choose a special "card" (action) for each phase. These cards give you various bonuses, but also have vulnerabilities (hopefully, you will be able to counter opponent's card with your card). Other that that, the battle is completely cinematic.
I must say I was quite surprised how nicely the "director" picks the cinematic shots, however the question is whether this won't get boring in the long run. But the largest battle I have witnessed was 2 ships vs. 2 ships, so larger battles may be very interesting 
The cinematic battles are also the only place (along with ship designer), where you can actually see your ships. The ships are quite nice, but definitely not X3 level (a few more polygons wouldn't hurt).
Conclusion
The game seems to have a lot of depth and so far I have only scratched the surface. So far, it was very pleasant surprise. The game is already extremely polished - I didn't experience a single bug and except the mouse scrolling issue, it feels like already finished product (it is more polished that most games in release state).
I definitely recommend this to any space strategy fan.
