In general, the simultaneous turns system works well, but as the game progresses it will begin to take longer and longer for each player's computer to process turns. Throw networking into the mix and you're looking at a lot of added time in between turns as the player count increases. If you can deal with that, you'll likely have a good time, but games even on the quickest settings will take many hours to complete.
Each player begins with around 40 seconds to take their turn, but this will increase as the game goes on. I wasn't the one hosting the game, but I hope there are options to tweak this clock as it can get a bit confusing to try and do everything you want to in a turn later on in the game, especially when you start dealing with barbarians and city-states. The depth of the game along with the fact that each unit must be moved individually means that turns will take longer than in previous Civ titles. Hopefully, additional multiplayer modes that don't rely on simultaneous turns are in the works.
The multiplayer interface is further streamlined from the already wonderful singleplayer UI. You'll have quick access to diplomacy options within a fold-able menu, without the need for fully rendered leader screens. Combat isn't animated and will quickly resolve. My concerns come from players interacting with each other simultaneously. The host of the game will process turns in the order his or her computer receives them. That means players that click first get to act first. There was one instance where I was able to "snipe" a barbarian camp from another player. He had dealt most of the damage on previous turns, but I managed to click first to get the last hit, giving me the free gold. It could get a bit tricky, especially during late game battles that rely on multiple units mixing ranged and melee combat.
There was also an issue of turns ending automatically if no moveable units were left, despite having potential actions in city management, diplomacy, or any other non-unit systems. This was forced through a server setting made by the host. If left on, players will have to utilize a workaround by keeping one unit active until all of his or her actions are complete. Very annoying.
The simultaneous system should create some exciting moments, but it could all fall apart during large battles. Sadly, the session didn't last long enough to hit this point in a game. I can easily see honor rules being created amongst friends to take turns during wars to ensure the combat plays out fairly for each player and not for the one with the fastest mouse hand. Honestly, this multiplayer mode is probably best used in cooperative games against AI opponents.
Civilization V will be released on September 21 for the PC. It uses Steamworks and can be pre-ordered in both regular and deluxe editions. These impressions are from a non-final build of the game.