If you enjoyed the Master Of Orion series, then you'll definitey LOVE Galactic Civ II!!!!
GC II is eye candy wrapped around hours of endless gameplay. I can see myself losing a full weekend playing GC II (I had to force myself to stop playing lastnight, I'm a family man). There's something addictive about turn based games that makes you want to click that "Turn" button one more time, and GC II is no different.
The level of detail in this game is amazing, yet not too over bearing. The number one feature of this game is the customized ship design. You can spend countless hours just designing your own custom ships, limited by your technology tree.
My only regret was not purchasing the retail box and purchasing the digital download only version. I may just go out and buy the retail box for the manaul :)
BTW... don't listen to anyone that says the game sucks because it doesn't have mulitiplayer. I guarantee that you'll be playing this game by yourself several years from now.