So I played my first multiplayer game, and it wasn't exactly pretty.
5 players on a small North vs. South map, and from the start it was extremely laggy. Sometimes the transition to the next turn took longer than the actual turns themselves.
Then the game hung up and forced two people to drop out. (Someone mentioned that we could all just reload from an autosave, but no one knew what he was talking about - apparently the game does autosave MP games but there's no way to save manually and no "load" option while playing MP.)
Then a third player dropped out, leaving me and one player who just happened to mass a legion of, well, Roman legions on my doorstep. An hour later of haphazard simultaneous-turn combat, with me franticly clicking on units and trying to get orders in before they get wiped out (and often helplessly watch as orders take 5-10 seconds to register, if at all), his offensive stalled enough for the two of us to say, ugh, it's past 2am, time to quit.
I'm left feeling quite ambivalent about Civ5. For all the great advances Civ5 makes, it has some deep flaws that probably can't be corrected in a patch. The computer AI does not wage war well, at least on the lower difficulties - it acts as if it can just stack of doom you, throwing melee units into fortified positions with ranged support. Meanwhile, the higher the difficulty, the more advantages the computer gets towards winning non-domonation victories, making war all the more enticing.
Playing against live people seems like a satisfying way to avoid AI issues, but even on quick the games take too long for casual play, people can drop out as soon as they start sucking, and simultaneous turn combat under heavy lag is frustrating.
One other thing that bugged me about MP is the unwritten rules - if you come across another player with one lone unguarded city 20 turns in, is it cool to storm in and knock that player out ASAP? Seemed like I would be forever hated if I played that ruthlessly. What about a player near me pouring everything into Wonders at the cap and forgoing expansion? If a player is 10-15 tiles away from you and manages to build Stonehenge, the Oracle, and the Great Library, shouldn't I just go ahead and take him out? There's no way they can possibly have enough troops to defend the cap.
I guess I should play more MP to learn the unwritten rules, but the lag and the buggy port badness of an already buggy Windows game are big deterrents.