Purchased the game yesterday. It is very similar to Civ 5, but is a little more fun to play (so far). I am especially fond of the elegant and lean game mechanics: No flimsiness and unnecessary features. Everything in the game has a clear, well-defined purpose. Having said that it's certainly not a "simple" or "primitive" game - there's a lot of depth to it. The game's intuitive and crystal clear UI is a big plus.
Research, improvements, expansion, and empire growth all follow the same 4x/Civ 5/Sins formula. If you've ever played these games, you will pick the game up in a matter of minutes.
The combat system is...interesting. I was initially put off by forum posts from alpha/beta testers complaining that the battle system is lacking. It is to a certain extent, but it is tolerable and IMHO satisfactory for the moment. From what people tell me, this kind of approach has never been attempted before so I am willing to give the game some slack in the combat department.
The developers seem to be putting a lot of effort into post-release development and patching. I don't doubt that the game will receive a lot of love over the coming months (years maybe?).
I purchased the game from Green Man Gaming which has a 25% off of everything sale. I am very happy to have snapped it up at a good price. I am enjoying this game a lot. I give it a solid 8/10 with a potential for 9/10 in the future.
If you like 4x strategy games, you will like Endless Space.