I voted "sometimes slow but ok."
I have 2 copies of the game running on 2 machines, and one performs a bit better than the other. The slightly faster system:
HP notebook running XP (media center)
P4 around 3ghz
Radeon x300 mobility
512 mb ram (will be upped to 1 gig soon)
And the slightly (very slightly) slower system:
Dell Dimension 8100 running XP
Pentium 4 (old model), 1.5 ghz
Geforce 6600 GT (128 mb)
712mb ram
(For the tech-heads: this is a first-model P4 system, which means that it is running the pricey and dead rambus memory. Also, the AGP port is 4x (AGP 2.0 spec), so the Geforce card is somewhat throttled in its abilities)
The intro movie runs fine on both systems, but the wonder movies are very, very choppy. I hope they fix that. I run the game at max graphics and 1024 x 768 resolution until around 100 BC and it plays very smoth. After that date, however, it slows down quickly so I set all the graphics options to low. That takes care of it, although I lose all the cool unit animations of course.
Personally, with all the horror stories about getting the game to work, I'm just happy I was able to get it running on both machines. But I am still hoping that the first patch is a doozy--Firaxis really needs to improve the performance of this game.
Edit: another interesting difference between the machines it that I get the diplomacy bug (where you can get the AI to give everything he has for peace) on one machine but not the other. The ATI machine is running a copy of the game I bought at Gamestop (not preorder edition), and it has the diplomacy bug. The other machine is running the preorder edition, but it doesn't have the bug. Very odd.