Don't Buy IT!
The first is the graphics requirements slow down the game, especially on larger maps, to make it virtually unplayable. The game pseudo-freezes three times per turn in the 1800s on a huge map. (I have a brand new system windows rating 7.4). The inability to properly run the game is a deal-breaker.
The second is similar to the first. The long off turn times make multi-player much slower and untenable.
The third problem is the game itself plays slower . Don't be surprised if you have 1 city and 1 worker still at 2000 BC.
The fourth problem is another huge deal-breaker. The new nationwide happiness system makes it impossible to have a large empire.
The fifth problem is the deletion of many interesting civ features: religion, corporations, spying, and culture converting. Other features are watered down: great merchants, great scientists, great artists, great generals, wonders, chopping and others.
There are other problems, but the above are the major ones in my mind, and at least one and four would make me strongly suggest DO NOT BUY.
I have uninstalled the game and will attempt to return/exchange it.
To be fair, some of the changes are interesting. I think the social policy system that utilizes culture is a great addition. I think the single unit per square feature is interesting.
But these features do not compensate.