It's a great series and Civ V is defintely a crowning achievement for it. Gods & Kings is a minor improvement imo, but last I checked they were only discounting it like 25% for the Summer Sale compared to 75% for all the other Civ V stuff...
Civ IV is leaps and bounds better then V. And G&K, even though I havent put in that many hours playing it yet, was a HUGE improvement in gameplay and replayvalue. I played tons of Civ V while I was in the navy last year, and it doesn't leave you craving for more the way IV did. The games play out rather similar after a while and if there are two capable players playing against eachother, more often then not the games will turn out as a stalemate and a race for scientific victory. G&K creates a much more diverse, more fullfilling experience.
Vanilla Civ V is a fun game, dont get me wrong, but G&K took it above and beyond what it was when first released.
Sent from my GT-I9100 using VerticalSports.Com App