I loved Civ II, liked Civ III but found it a little shallow and so I missed out on Civ IV. I bought Civ V when it came out and although I found it fun to play, I did have some graphic issues with the game that impaired its enjoyability somewhat and again found the game a little shallow in depth until the latter stages of the game.
This expansion of the game has really rectified that. Religion adds much more interest to the early game and the addition of new Civs and City States was a welcome breath of fresh air too. I am still only playing my first Marathon game with the expansion, but it has been a much deeper and more enjoyable experience than the previous vanilla version so I highly recommend it even if you played Civ V and were not overly impressed. Graphic issues seem to have been ironed out and the game loads much more quickly and runs much more smoothly now as well.
Tactical combat is much more interesting now with the new combat system. The addition of Great War units was a good one as well, as I felt the previous vanilla Civ V rushed too quickly from Napoleonic forces to WWII era weaponry.
Diplomacy seems to make a little more sense now too.
Bottom line: if you love Civilization, this is the one to play. I think all of those people who preferred IV to V may be pleasantly surprised with the new changes, but having never played IV, I can't say that with certainty. Just seems that the Civ team listened to criticism and put back into the game that which had been taken out.
The only thing that sucks about this game now is how tired I have become from late night "Just-one-more-turn-itis"