The short of it: This game is really good, BUT ONLY WITH THE EXPANSION! DO NOT BUY AND PLAY THIS GAME NAKED!
Unlike a lot of 4X games, Endless Space is more about building your empire than about developing your colonies or really fighting wars. Not that you won't fight wars, the AI is usually vicious enough to wait and plot and it will build up an overwhelming force based on your existing military might before waging war on you. And it WILL do that. Every single chance it can, unless it can't, it will do that. Never think it won't, since it has no reason not to.
We'll get more into that later, but right now let's focus on the empire-building. Building up the empire is the primary focus of this game. Oftentimes if you look at too little at a time, you'll tunnel vision yourself into a corner when you should be keeping yourself as distant as possible. A prime example of this is expressed in happiness. The morale of a system and its subsequent production bonuses are NOT more important than the overall happiness of your empire. After all, a 15% production bonus across 4 stars is better than a 30% bonus in just one. If, when settling a new system, you have a planet with lots of production, or a planet with a high amount of happiness-boosting, you pick the one that makes your empire happy, or you lose the game.
Now then, let's talk about war. War is problematic in this game because A) it's inevitable, and B) it's boring. The game takes a very hands-off approach to combat and in the vanilla game technology is both everything and a linear arms race. The guy with the most ship tonnage and the best weapons will always win. You can employ tactics cards, but these will only have a minimal impact on the outcome. This leads to a very boring experience, and can force you into certain types of tech progression and thus certain modes of gameplay. To deviate from established doctrine is to lose in the vanilla game.
DO NOT PLAY THE VANILLA GAME. Instead, do yourself a favor and get the Disharmony expansion. The Disharmony expansion does away with about 75% of the weapon-tech in favor of just four generations of guns. Each is a sizable improvement over the last generation, and keeping up the with the AI is definitely important, but with tactics, ship design, aimpoint selections, and formation management you can steal victory from the jaws of supposedly assured defeat.
Endless Space encourages you to use your distant veiw not to mess with the minutae of your own empire, but rather to mess with the other guys. Through trade, trade routes, blockades, invasions, explorations, treaties, and wars you are charged to take away the choices of the enemy for as long as possible while you improve your own choices. How you do this is entirely up to you, and Endless Space: Disharmony actually allows you quite a bit of wiggle room on how you do it.