After having spent about 15 hours (according to Steam) on Civ 5, I have some more to say.   
 
First off, taking enemy cities is difficult enough to require planning and strategy, but not because the owner of said city has garrisoned a dozen units inside it.   Rather, cities themselves can defend now, and bombard from range.  Combined with the ability to garrison one unit inside, they can be quite formidable.  I've thrown four units at an enemy capital simultaneously and come away empty-handed.  This is good, because the days of mammoth empires of a dozen cities or more are effectively gone.    
 
The reason for this is that the maintenance and happiness costs of more than six cities or so quickly become prohibitive, and I am thankful for that.  In Civ 4, it was trivial to build an economy capable of supporting as many cities as one wished, and since happiness was calculated on a per-city basis, all that was needed was to address happiness problems in each city as they arose.  Now it is civ-wide.  The net result is that an empire of 3-6 cities seems to be what most AI civs end up with, and what I found most sustainable as well.  This is far more manageable in the end-game.  
  
Encounters with enemy units are more realistic.  Not only could I not imagine a spearman taking out a tank anymore, it's cause for severe worry when a warrior unit meets a barbarian spearman.  There's a chance my warrior has enough xp and superior positioning to come out on top, but I'd better be real sure.   
 
Thankfully, however, the AI is stupid.  This is nothing new, but without the crutch it used to have of huge stacks of defenders or giant empires, it is very plain just how simple it is to outsmart.  I'm not sure there's a good solution to this.  From a programming perspective, it's a large enough problem space that it would take a much more sophisticated program to be able to traverse enough of the possible solutions to each scenario to put up a good fight against a human.  In other words, I wouldn't expect a fifty-dollar piece of software to be able to adequately comprehend the complexity of the game world it must play within.  
 
Many of the leaders from past Civ games have returned, and it is good to see that their "personalities" haven't changed.  Montezuma is still a homicidal maniac, and Gandhi would still rather give you his sister than go to war.  The new leaders seem to fit in well by and large, although their characters aren't as well-developed as the old guard.  It is a welcome new addition to have them speaking their native tongue in diplomatic interactions.  Others have complained that diplomacy is damaged because there are no longer any numbers indicating how a particular civ feels toward you, but I'm finding that this isn't all that necessary.  The expression on your opponent's face and his words at your arrival are generally telling enough. 
 
The focus of the game as a whole is decidedly military.  There are victory conditions for those focusing on science or culture or diplomacy, but the systems supporting those are ... not as robust.  In Civ 4, it was possible to beat the game by making enough cities with enough cultural influence to dominate the globe.  A particularly influential city could effectively annex a lesser enemy city without firing a shot, simply by swallowing it within cultural borders.  This mechanic seems to be gone.  The space race is still here, but I have to admit I've never found it all that compelling in the past and haven't tried for that victory yet.  I can't imagine it's all that different, given what I've seen so far about the tech tree.  
 
All this being said, I am satisfied so far.  I plan to continue playing this for many hours, and am curious to see the direction the game heads with future patches and expansions.  

After having spent about 15 hours (according to Steam) on Civ 5, I have some more to say.   
 
First off, taking enemy cities is difficult enough to require planning and strategy, but not because the owner of said city has garrisoned a dozen units inside it.   Rather, cities themselves can defend now, and bombard from range.  Combined with the ability to garrison one unit inside, they can be quite formidable.  I've thrown four units at an enemy capital simultaneously and come away empty-handed.  This is good, because the days of mammoth empires of a dozen cities or more are effectively gone.    
 
The reason for this is that the maintenance and happiness costs of more than six cities or so quickly become prohibitive, and I am thankful for that.  In Civ 4, it was trivial to build an economy capable of supporting as many cities as one wished, and since happiness was calculated on a per-city basis, all that was needed was to address happiness problems in each city as they arose.  Now it is civ-wide.  The net result is that an empire of 3-6 cities seems to be what most AI civs end up with, and what I found most sustainable as well.  This is far more manageable in the end-game.  
  
Encounters with enemy units are more realistic.  Not only could I not imagine a spearman taking out a tank anymore, it's cause for severe worry when a warrior unit meets a barbarian spearman.  There's a chance my warrior has enough xp and superior positioning to come out on top, but I'd better be real sure.   
 
Thankfully, however, the AI is stupid.  This is nothing new, but without the crutch it used to have of huge stacks of defenders or giant empires, it is very plain just how simple it is to outsmart.  I'm not sure there's a good solution to this.  From a programming perspective, it's a large enough problem space that it would take a much more sophisticated program to be able to traverse enough of the possible solutions to each scenario to put up a good fight against a human.  In other words, I wouldn't expect a fifty-dollar piece of software to be able to adequately comprehend the complexity of the game world it must play within.  
 
Many of the leaders from past Civ games have returned, and it is good to see that their "personalities" haven't changed.  Montezuma is still a homicidal maniac, and Gandhi would still rather give you his sister than go to war.  The new leaders seem to fit in well by and large, although their characters aren't as well-developed as the old guard.  It is a welcome new addition to have them speaking their native tongue in diplomatic interactions.  Others have complained that diplomacy is damaged because there are no longer any numbers indicating how a particular civ feels toward you, but I'm finding that this isn't all that necessary.  The expression on your opponent's face and his words at your arrival are generally telling enough. 
 
The focus of the game as a whole is decidedly military.  There are victory conditions for those focusing on science or culture or diplomacy, but the systems supporting those are ... not as robust.  In Civ 4, it was possible to beat the game by making enough cities with enough cultural influence to dominate the globe.  A particularly influential city could effectively annex a lesser enemy city without firing a shot, simply by swallowing it within cultural borders.  This mechanic seems to be gone.  The space race is still here, but I have to admit I've never found it all that compelling in the past and haven't tried for that victory yet.  I can't imagine it's all that different, given what I've seen so far about the tech tree.  
 
All this being said, I am satisfied so far.  I plan to continue playing this for many hours, and am curious to see the direction the game heads with future patches and expansions.  

I've yet to meet a peaceful Ghandi in my games. He's been an utter warmonger in two of my games. No matter what I did, he wasn't pleased,  and I had to resort nuking one of his cities to glowing dust before he finally got the message that  'an ounce of practice is worth more than tons of preaching'.

I think the AI is complete and utter shit for a $50 game. You're right, to make an AI that could fully comprehend everything that is going on and make the most effective decision would be insane and out of a $50 game, but even so, the AI is pathetic. It makes the game a lot less fun when the AI does completely nonsensical and stupid things. I will say there are times where I will meet nations that actually make rational decisions throughout the game but not often. Also, I dont agree with the whole "amount of cities" thing. I just feel like the amount of cities you have is all relevant to how you're trying to win the game and what is happening in the game at the time.

Meh, I really want to enjoy this game...and I love it when it works. 
 
My major problem are the crashes, I'm okay with everything else.