The expansion for Civ V has been out for awhile but what with a minor addiction to D3 I never picked it up. Since my interest in D3 has waned very dramatically recently I figured I would buy Gods and Kings and see what changes came to CiV. I must say I am pleased with the changes they have brought in at first glance. Many of the things that I fixed or tried to fix in my CiV mod have been implemented in G&K like reducing the power of Great Scientists and improving little used military units; I can't really take credit since many other mods did the same thing but I do like it when things I did end up in the game.
The addition of religion to the game works pretty well mechanically I think. You can still play the game pretty much as you always have if you really want to but there are a lot of new options and styles available when you dive into religion. The fact that I could play the way I used to but dip into religion carefully when it seemed useful is a great way to introduce a new mechanic; it means that players won't be overwhelmed with new choices and that the balance that has been achieved is easier to maintain. When a new expansion adds completely new concepts to the core of the game it often ends up being either incredibly badly balanced or very confusing to current players or both.
Unfortunately with all of the really good changes there are still some things that stand out as not working well to my mind. There are still buildings that are fantastically bad and which will never, ever be built by a human player playing optimally. Having buildings that are mediocre but which have situational abilities is fine since we do want players to make meaningful decisions but I don't like it when you make a decision precisely once and then never revisit it regardless of what is going on in the game. I am still going to build a mod and try to correct the flaws I see in the game but I suspect that it will be a much lighter touch this time as I make minor tweaks instead of major overhauls.
If you liked CiV but haven't picked up G&K then I would recommend you do so. It gives a whole lot of new life to the game and makes it feel very much new and shiny again. The complexity level has gone up a notch or two but I think that this is a good thing; at this point in CiV's life cycle it is mostly the hardcore players who are still going on it and they like a little extra meat on the bone.