Gamespot: 9.4 (out of 10). "If you have even a passing interest in strategy games, world history, or getting less sleep at night, you owe it to yourself to give Civilization IV a try."
IGN: 9.4 (out of 10). "The only reason to stop playing Civilization IV is to tell other people just how good the game is." (That's been my experience, too! - ed.)
Gamespy: 5 stars (out of 5). "Civ IV gives the classic turn-based strategy franchise a much-needed overhaul, with streamlined gameplay and a great multiplayer mode."
1Up: 9.0 (out of 10). "Civilization IV's thoughtful turn-based take on conquering the world will have you under its spell late into many a fall night oblivious to the changing seasons outside."
Yahoo! Games: 4.5 stars (out of 5). "The latest in the series is its greatest yet."
Metacritic: Critics: 94 (out of 100), "Universal Acclaim". Users: 9.7 (out of 10). Several quotes, my personal favorite is: "If you're an AOE fan but never thought you had the brain for CIV, then this is for you." I think they meant that in a more complimentary sense than it came out.
My very next post will have lots of info on my current game. I'm holding off until my official review, but I'm inclined to agree with this range of scores, with the important caveat that your experience will be less if your computer cannot run the game.