When it shines, it's a beauty, but when it stinks, it kills everything good about it.
I want to like this game. It has that "one more turn" factor. Visually it's well done, and the UI is pretty clever with most complex orders only a few mouse clicks away. But then you find reordering production queues to be unusually clumsy. Most of the game is like that. A few solid hits then a terrible miss.
The races are good, well designed and presented, each with their own strengths and each requiring a slightly different set of priorities to succed. But then the difficulty modifiers kick in and the whole race balance goes out the window. Enemy AIs get bonuses to production NOT intelligence and you fight races that can outperform what you could do with them.
Then the AI itself shows irreconcilable stupidity. If i could tolerate 6-7 dumb things the AI does per turn, it would do 10. All too often the AI fleets that could potentially beat you choose to flee (but waste your time, attack, or leave ships behind that blockade you), while i have had one completely unarmed and undefended fleet of invasion ships try to fight back (with tactics, not weapons ofc) and even somehow managed to flee with a few survivors.
The AI can sometimes use tactics wisely and from what i saw has no knowledge of your own tactical choices, but the battles are a lottery decided far far ahead, back when you designed the ships. Some survive, some have no chance, and if your ships are very good, then AI will either allways retreat, or allways die, depending on whose territory you're fighting in. Sometimes the very outcome of a war or even game can hinge on the AI doing something remarkably stupid or you failing to understand some of the BASIC fundamental rules of the game. That... is beyond bad, that is awful.
The diplomacy model is one of the better i have seen. Most of it is really good, except of course the one part that kills it completely. It's all meaningless because ultimately your empire exists and therefore it would be better for a rival to take it all away from you. You're the weakest empire? Then die. Wait, you're strongest? Then die. You're in the middle, and a long term trading partner as well as supplier of strategic resources, benefits that far outweigh the problems caused by close borders (and you can see the exact numbers too)? Guess what? Today is the day none of that means anything. Prepare to set the relationship score to -200. And die.
Colonisation and expansion is arguably the strongest part of the game. Exploring the galaxy, triggering some halfhearted random events, scouting good spots for colonies. First contants. All good. But. The game actively penalises you for each and every system you own. Wheter you colonise it or capture it, just having a system increases the penalty higher and higher. Eventually it becomes high enough that it starts to hurt morale and that in turn drops production, research and cash flow. Yes, by colonising a system at a wrong moment, you can drop overall empire wide production by some 10-20%. Does that make any sense?
Yes, there are technologies that reduce the penalty, and yes, it's there to counterbalanc expansion but i am here to play a game, be emperor of space, not to oversee active game modifiers. They should be invisible and subtle, not hit me in the head. When i deliberatly skip average systems that are still well positioned, all just to avoid getting the expansion penalty down the road, i think i am playing a flawed game.
Combat is neither good nor bad except when it's completely illogical. Designing the ships properly takes some experience and knowledge and if you do a good job your fleet will usually serve you well and survive for a long time or you will produce enough replacements for losses to not matter much. The main problem with all the tactical options, counters, different ranges, weapon optimisations, support ships, repair capacity, fighter/bomber cover, is that you very often feel like you just can't take your ships and tell them to "$#%^&* go and kill the enemy!", the MOST BASIC OF PURPOSES for your fleet, but somehow it gets lost in the myriad different things you have to tweak before you can fight. Long story short, i know what i want, the game wont let me, or i have to obey some rule/design feature/balance mechanic.
So while it starts pretty well, and has enough to be fun for a while, it eventually becomes a chore to try to complete a game. The AI has too many choices to make and inevitably it makes poor decisions (compare to SOTS' very basic but lethal AI) and all the strengths of the game are drowned out by the weaknesses.
The best turn based space 4X games have already been made. Master of Orion 2 and Sword of the Stars 1. Maybe even Birth of the Federation, though i am biased about that one. Other games of the genre try to do things differently which is unfortunate because sometimes imitation is the wisest form of flattery. Case in point: Xenonauts. For ages developers tried to recreate the perfect succesor to XCOM - UFO Defense. UFO: Aftershock is 99% as good, and UFO: Alien Invasion is an extremly good remake (but still unfinished). A slew of other games missed the mark completely or admittedly used the old game as a springboard for a new experience. Then Xenonauts imitated the original and surprise, surprise, it hit the bullseye.
Ultimatly Endless Space isn't solid enough to satisfy demanding 4x players, and it isn't "mainstream" enough to captivate the casual audience. So it ends up in between. Better than most, but not good enough.