The Civilization V expansion, Gods and Kings, is a very nice EXPANSION to the game. How rare these days… A nice overview of the two main additions, religion and espionage, can be found at Mashable.
I’m only one game and 300ish turns in, but I like what I’ve seen so far. The new take on religion seems far more ‘natural’ than what was done in Civ IV, as well as being more gamey. Getting to design your religion’s bonuses not only makes the whole thing have more impact, but nicely skirts the issue of making one religion ‘better’ than another in terms of bonuses. Now if you make Islam awesome and Judaism suck, you are the terrorist and not the devs.
While religion is an important factor, espionage seems to be pretty minor. It’s allowed me to steal a few technologies, and see some cities, but it’s not been nearly as important as religion. Perhaps further into a game or with more experience this will change. That said, it’s a welcome layer to an already deep game, so its addition is positive overall IMO.
I’ve also noticed a fair amount of UI improvements, all of which (so far) are very nice. Performance-wise the game is better, although I’ve never been one who had major issues with the original game.
More civilizations is also always nice, and I feel like the new additions expand the strategic options well, rather than just adding civs for the sake of adding them with generic or non-unique units/buildings.
If you are a huge fan of Civilization, I’d recommend picking the expansion up. It more than adds enough to the game to rekindle interest and get you hooked. If you can wait, and don’t have Civ V with its DLC, indeed wait for the inevitable Steam bundle to save some cash.