I've just finished my first multiplayer game of Civilization 4. Me and Dan won a domination victory on an inland lake map.
My opinion of the game has increased a hundred-fold. The game truly is designed with multiplayer in mind, and played flawlessly through the game me and Dan played. This game for multiplayer is 100x the game Civ3 Conquests ever was.
First off, it works. Connectivity is flawless, even through NAT and firewalls. It's stable too; no out-of-sync, only the occasional delay waiting on players, and we ran through an entire game in three sessions. Only one crash, but it was Dan's end, and he crashes in most games. Even at endgame, while our computers and network were definitely earning their keep, the game was certainly quite playable.
Second, it's polished. Gamespy lobby works perfectly. Even loading works well - you get to choose which slot to fill in the game loading, and you can set a password to prevent others from choosing to resume your empire! Game resumes perfectly without a hitch. Even better, during Dan's only crash, it brings up a VOTE, asking whether the players want to save, and whether they want to quit, or continue. Yes, a savegame at the very instant of the crash. Hurrah!
Third: teams. We play co-op mostly, and teams just make it so much better than Civ4. No more "finding each other" and "calling techs". If you're in a team, you're in a permanent alliance through the game. Automatic open borders, tech shared (current research is also listed in the player list, communciation shared, visibility shared, even wonder benefits shared. War status shared, which can be a pain at times, but probably makes sense. Even victory conditions shared, so the co-op team can actually win as a team.
In Civ3, we never actually FINISHED a game successfully, so we never even considered victory conditions.
One observation though: the game seems very tuned to space race victories. Even with me and Dan basically in a constant (always victorious) war from the middle ages onward, it was still 60 turns to finish when we got a domination victory. We never really slowed down for anything; it was a constant push around the loop, and us never getting pushed back. The other forms of victory seem rather far-fetched unless you're planning for them from the start.
While I stand by the fact that there's definitely both usability and actual bugs that need to be addressed, they seem to pale now in comparison to the joy that is actually playing and enjoying the game. I can't wait to make Civ4 a regular experience again.