Before I even get started, I should preface this by saying that I haven't yet played even 1 turn of Civ5 SP. Immediately after work my friend and I installed and began playing MP duels. Furthermore, I have not yet played past the Medieval era. But after a good handful of 1v1 MP rounds this evening, here are my thoughts.
Let's start with the good:
Civ5 is fundamentally a good game. Hexes and 1UPT are really what set it apart from previous iterations (finally we have meaningful decisions to make in warfare)! The myriad ways in which combat decisions can make or break your assault (or survival therein) have been discussed at length in other reviews and commentary, so I won't detail them here; just play a few games and see for yourself.
Some of the simplifications that others are griping about are actually enhancements in my view. I honestly hated a lot of the micro-management and clutter in Civ4. Was it really enjoyable building roads in every tile, aqueducts in every city, and moving dozens of units all over the map, onto and off of boats?
And, of course, the game is very pretty; not due in small part to hexes.
Now the gripes:
This game is not ready for release. I routinely told units to move two tiles, only to watch them move one and then be prompted to move them again. On a few occasions I noticed that I was able to move a unit even though the "next turn" indicator was present. And where is the "shuffled" map type? Strangely, the game room screen (after host setup) shows "map type: random" when you select "continents" as the map type, but only if your map size is not set to random (?).
The tech tree is pretty narrow and doesn't allow for a lot of strategic variety. As one reviewer mentioned (I can't remember which one), Great Library rushing seems to be *the* strategy to pursue in most/all cases.
Barbarians are irritating, but not much else. Tactically, they are low-functioning retards. Nonetheless, they seem to appear out of nowhere and relentlessly cause messes for you to clean up. One barbarian spearman wondered into my borders and pillaged a mine in a single turn. That particular problem could be remedied if the view radius around your borders extended two tiles. Okay... fine, I could have placed units on my perimeter to scout out barbarians, but either way would my encounter really have added anything meaningful to the game experience? Nah.
Okay... back to the game now :-)