GoodGamer: GalCiv2 with all of the expansions, or at least the first expansion, is awesome and highly polished. And still being supported (new patches)! Go to galacticcivilizations2.com
MoO3 is basically well-intended garbage that tears at your soul that it could have been great.
Basically, what he said.
I gave MOO3 a good hard run. The game basically sucked. You fleet designs were REALLY restrictive. If you want X amount of ships, then you must have 5 classes that all fit this cookie cutter, and if you don't match it, they cant be made a fleet and can't fight. Also, missiles=win, no if's and's or but's. The racial interaction was soso, how the building system worked was just inferior to MOO2, and regardless of what you wanted, the local governors worked against you. It looks as if they went out of their way to make it NOT fun.
GalCiv2, is a great game, not awesome IMO, but great. I LOOOOOVE the ship design system in it, although I wish there were more MOO2 like mods for them, like HyperX Capacitors, Range Master units, High Energy Focus etc. I rather liked the Armor vs Shields vs AMS systems, really excellent idea. The tech tree was solid, while more realistic than MOO2 in as much as it was non purely non linear progression, I do still have a soft spot for MOO2's way of doing things. Also, the commentary on the tech tree is frequently rather amusing.
If you dug the MONSTER fleet aspect of MOO2 late game, GalCiv will leave you wanting as the fleet sizes are pretty restrictive. Very doable, and any config you want, but it is limited to your command points.
The economy in GalCiv is a lot trickier than any of the MOO games. At times, overly seemingly meaningless events, your economy tanks and can be a pain to get it going again, and some of the info is counter-intuitive in as much as it isn't always easy to find, or to get all in one place, which if you make a mistake in the learning curve, it can be a challenge to find what is wrong to know how to fix it.
3 complaints about Galciv2.
1) No co-op. I know to some, who cares, but me and a friend from Highschool have been playing MOO2 hotseat for 15+ years, and would love that, or even internet play. Don't care about PVP, but co-op in these games is a blast, and GalCiv does not have it.
2) You cannot control your ships in combat. It crunches the numbers and gives a video of it. I would love tactical combat that gave a movie option after..... but there are fights I KNOW I could have won, but the AI couldn't.
3) Starbases are cannon fodder for the most part. You can gear them up and make them soso, but they are basicly social centers and static buffs. I prefer the MOO2 way of a base is a tough ship, and fights along with the fleet and planet.