Been playing and largely enjoying Civ 5.
High points:
Great level of polish.
The Civ's bonuses have been tweeked, but so far seem pretty balanced. They are also more interesting by far than the bonuses in Civ 3 and 4.
New hex maps and tactical combat are awesome.
The mechanics in general seem more refined. Culture, boarder growth, etc work well.
Game feels a little more dynamic with city-states and all they entail.
Low points:
I have a fairly high-end computer (GeForce 260, i7 2.7GHz chip, 12 gigs RAM....), and on almost highest settings I find the game chugs. Talking heads/Diplomacy screens with freeze the game for 10-15 seconds, and stutter for another 10-15 before they're working. If I reduce the video settings to minimum, I get a smooth experience, but when my computer exceeds the reccomended requirements by an order of magnitude or two, and still has issues with the game at medium settings....
Too few Civs
No scenarios. I suppose these will have to fall under "mods," but they could have easily shipped the game with a few set-pieces, like they have done in previous versions.
A real pain in the ass to alt-tab out of. I like to switch between games and other applications, but Civ V hates handing the reins to any other program while it's running.