Bit more of a performance hog than previous Civ's and I've got the stutterring movies playback that's been plaguing some other folks, but I've turned movies off for now and am still enjoying. I'm sure some of these problems can get ironed out in a patch--in fact, Firaxis built a patch updater into the game!
I'm an English perfectionist so I noticed a few misspellings and some mismatches between what Sid says and what dialogue is printed (in the tutorial), but the new game mechanics are a great enhancement.
Civ 1 got us hooked and the cartoony world leaders were fun. Civ 2 upped the strategy pieces and introduced an isometric format as well as the classy touch of wonder movies. Civ 3 introduced more strategy components and a clean interface, but somehow didn't engage me as much as Civ 4 does.
Somehow taking all the fun parts of the previous Civ's and taking out the boring parts, Civ 4 reminds us once again how great a GAME it is. Colorful, with history again made lively, and stable/playable for the most part, I agree with the reviewers who gave this one a "must buy!!!" I can't remember being this hooked since putting Civ 1 into a 3.5 floppy drive...