I've experienced both minor lag problems and strange graphical issues (I have a GeForce FX 5200 card), but nothing you can't overcome with some tinkering around. It really appears to drain a lot of memory, though, so you'll have to keep your sessions moderately short. When it comes to gameplay I've found it to feel a bit short. I reached the industrial era pretty quickly, and it feels like units become obsolete even faster than in previous Civs. I must admit that I haven't had a single war with more than the occasional skirmish, however, so that detracts a bit from time consumption. Overall it seems cool, but not quite what I'd hoped for.