Just got done playing a 5 hours session.
I played as the Washington (americans) on a huge continents map type. Normal speed. 12 AI opponents and 24 city states.
First impression for a second was "oh no this looks like the console interface" few seconds later "thank god I was wrong"
Much like civ 4 this game is a blast to play. If you've played any civ game you can jump right in. The interface is alright. Not great but its not horrible either.
The graphics are extremely good looking. I was atcually sad to watch my industrialization of my lands. As the rolling forests and verdant hills turned into marketplaces, mines and farms.
That being said I have had no lag whatsoever. I have everything cranked up on high and the only time is slows down is when I zoom out and the giant continent I am on shows up in all its glory. I am on around turn 250 and with that many ais doing things turns can take like 20 seconds to load. Not bad. I have patience so it doesn't bother me at all.
The combat mechanics and a lot of other things are changed but no so teeribly much that it feels like a different game.
The biggest difference is the stack of doom. At first that was my beef before the game came out, no stack of dooms for me to march with to my enemies cities. After playing though, this is much more enjoyable. You cant just amass and spam some troops and roll over. Numbers still help, but so does positioning, flanking. The combat, in my opinion, is better for it. Its more tactical. They also improved that tool tip that popped up when you were about to attack. It really tells you whats going on. There is still some wiggle room and a little randomness but I like that. Thats how life is!
The City states are neat and add a little something extra to the game. They are not awesome but they can be usefull. early on they are too expensive to court or they demand to much (like asking you to wipe out another city state) but later on you can have a handful and the AI reacts to that. They either make nice with them or start gobbling them up. The bonuses are neat and get better as time goes on (as you reach different ages)
All in all I think its a great game. I have had zero crashes or other technical problems with the game so far. Only 5 hours in though. I have around 5 AI on my continent. I have no idea where everyone else it at.
If you liked previous civ games or like turn based strategy try it out. Between this and Elemental my free time just took a hit.
PS. The downside to Civ 5? No fall from heaven...
EDIT:
Someone PM my asked about my system specs I use to run the game.
Windows 7 Ultimate 64bit
8400 core 2 duo 3.00Ghz.
8 GBs Ram
2 (in sli) EVGA 8800 GTX 768mb video cards (no where near top shelf anymore but 2 of them get the job done!)
When you start up the game it asks you if you want to run direct x 9 mode or direct x 10/11 mode as well. If you have XP you are stuck with 9. There is not a very noticeable difference in the graphics between the difference, but the 10/11 does look a little better and (Seems too for my brief tests) play better.
As far as Steam goes, I'm not a fan of it. but it doesn't interfere with the game at all. Steam runs. Then the game runs. Done. I don't like having to use steam but like any game I have had with steam, I have had no problems, or steam acting stupid. Just my personal experience.