Way worse then Civ IV. Brings nothing special, new or exciting to the scene.
Civ V doesn't provide any of that epic feeling when you grow a nation from the ground up. Developers have to understand that strategy games aren't about graphics and shiny explosions. Although some have managed to combine performance, great gameplay and shiny graphics (Supreme Commander 2, Company of Heroes etc).
I like the new combat system and one unit per tile thing because it makes the game more strategic, reminding me of Elven Legacy although it makes ranged units very dangerous.
Currently the city-state system is very simplistic and unbalanced. This is clearly a rip-off of the minor races system in Galactic Civilizations. But in GalCiv, minor races are part of the game, you can interact through diplomacy with them, they develop on their own, they feel small but powerful. In Civ V, they seem like little handicapped civilizations that bug you to destroy some other city or protect them from barbarians and so on. I like that they provide you with bonuses, units and their resources, but it's stupid to always have to give them money because your relations always decay by default, it's just unrealistic.
Diplomacy is a joke. They made a separate diplomacy screen with fancy 3D graphics for all the leaders, and they all talk in their native language. Who cares about that? I liked having tech trading as an option in Civ IV, but it's gone now. Instead you have the options to make research treaties (again GalCiv) which in return for money, provide you with a new tech at the end of the treaty. Also other treaties seem to dissolve by themselves without you knowing why. It's all very confusing. Also I didn't see any indication about how much other civs like me. Another thing, for example if I want to make a deal to receive money from a civ in exchange for a luxury item, I have to click on each one, which loads up new 3D scenes (which can take like 500ms) and click Trade to see if they actually have any money to trade. It's annoying. Not to mention that diplomacy is still waaay too simple for a 4X game this old. Did Sid ever play Galactic Civilizations 2 Ultimate Edition?
Gameplay is simplified. No more forms of government, which were to some extent quite realistic in Civ IV. Instead, you have this Policy thing where you chose national policies which provide bonuses and abilities to your nation. I like that, it gives each civ personality, but is not reflected strong enough. I would like to see a militaristic civ receive some huge bonus at the end of the tree and so on, that would make it more worthwhile.
Speed and performance is appalling. And they dare state this game can run on low-end machines. Yes, it can run on Intel i7s with integrated graphics card, because these equivalate to nVIDIA's 9400GT. I have a Core 2 Duo @ 2.4GHz, 4GB of RAM and a 9600GT and playing it at lowest settings is still a nightmare. I don't care that workers make little sparks go out of the ground when they work, I just want the game to run smooth, but it doesn't. The game has to load textures everytime I change the zoom level or move to another area of the map. The performance gets worse and worse with every turn. I understand this is a new engine and it needs optimization but it's a strategy game ffs!
Conclusion:
-no epic feeling when playing
-very very slow gameplay, performance issues, lags, interfaces lags, and I just don't see the point of it
-city-state system not thought through
-stupid diplomacy with lack of feedback
-good stability, the strategic view would crash the game every time though
It just isn't worth it. Civ fans stick with Civ IV with Beyond the Sword expansion. 4X fans try Galactic Civilizations 2 Ultimate Edition, it's a complex and polished Civ-like game in space, also turn-based, very complex diplomacy, very complex research trees unique to each race, you are able to design your own ships in great detail and it is said to have the most advanced AI in any strategy game, without it cheating to have the upper hand. The same people that made GalCiv also made Elemental War of Magic, also a 4X game, but in a fantasy setting. I'd give that last one some more time to mature though.