Big thumbs-up from me. I'm having a blast with it. Glad to see that a majority of voters seem to agree with me.
I loved Civ 4, but I already own that game, so I'm glad Firaxis has given us something different. I love the hexes and 1upt; it will be tough to go back to squares and stacks of doom after this. City-states are a cool new feature that I appreciate more with every play. Social policies add a second tech tree, but one that you can't possibly finish researching; this makes culture more interesting than just the amassing of 50,000 points. The new happiness system and the changes in tile improvements mean we have to learn new techniques for specializing cities (which is at least as important now as it was in Civ 4). The foundations of any good Civ game are still here: building cities and buildlings and Wonders, agonizing about guns-vs-butter, managing tradeoffs between expansion and happiness, etc.
For me, the various new features more than make up for what's been left out in this version (e.g., religion, health, espionage, corporations), not all of which I miss. I wouldn't mind if some of those features (some of which were in BtS, not vanilla Civ4) make a comeback in an expansion, but for now I'm having a great time with Civ 5 as is.
Sure, it's not perfect. The AI needs help navigating the new grid, but they've already started to patch it, and besides the old AI wasn't Deep Blue either. But for me, the proof is in the pudding -- many many hours of enjoyable Civ goodness. Back to it now.