Obvious comparison is obvious. 5 is a blend of Civ4 and CivRev.
Vanilla Civ5 blows Vanilla Civ4 out of the water in every possible way.
Vanilla Civ5 does not compare favorably to Fully Expanded Civ4 (Warlords+Beyond the Sword).
Civ5 is great, I play the out of it (you know this), but there are two key missing features like Vassal states and finely detailed diplomacy options that I sorely miss.
But the positives far outweigh negatives, IMO, with additions/modifications like Great People actually being...well, Great. That a streamlined economic system doesnt really allow you to expand beyond your means to afford it (in a way). That the AI can no longer swap techs behind the scenes creating this insurmountable gap between you and them unless you play the same game with some other AI (hated that in 4). Now people can enter into Research Agreements that award a random to each civ for a gold cost and waiting 30 turns. Best thing about it is that every Research Agreement is announced to the world (no subversion). The Cultural path is FAR better. Social Policies are the Whoop-Daddy no matter what your style of play is. City states took me awhile to warm up to, but theyre a nice addition in smaller doses than the defaults (large map = 18 City States (?), I turn that down to 10).
Less important aspects: The game looks 100x better in every aspect. The music and sound is varied, unique and appropriate for the moment (peace, golden age, war, imminent defeat). The leader animations are pretty ing cool and its nice to hear them speak in their native tongue (Montezuma and Nebuchadnezzar are my fav's).
Earlier in the thread I went pretty far into the combat in Civ5. Needless to say it is the most defining difference between the two games and that difference is not good, its phenomenal. Actually have to use foresight to plan an offense/defense, strategy on exact goals of said campaign and what to do with what you conquered and tactics up the ass on a unit-to-unit level. Awesome addition and the best selling point for the game and its future, IMO.
Soooo, yes, I say its worth your purchase. If you liked Civ4, I would bet you will like Civ5 after some getting used to it. Just like 4, dont get all comfortable at lower difficulty levels because they do not indicate (nor teach) the game properly. Playing anything less than Prince/Monarch would make you think the AIs are dumb, combat is one-sided and the game was just too easy.
It is, most assuredly. There is a certain amount of gratification you get from beating 5-1 odds in open war because youve properly teched and assembled your armies with a varied spectrum of soldiers.