I've played a couple hours of civ 5 each night the week it came out. I played civ 4 EVERY WAKING MOMENT the week it came out.
The weird thing is, for those of us not content and finding obvious flaws, it is so amazingly mysterious why people would value 5 over 4. Its like someone driving up in a tricycle and claiming it's better than their car. Y'know, it just needs a few patches, and a fourth wheel from an expansion.
I'm the opposite. I've been far more enthralled with Civ 5 then I was with 4 (which I still loved by the way).
Yes, the AI needs to be improved, and balancing tweaks need to be done, but the underlying design in Civ 5 is fantastic and I strongly feel that it's a beter game then 4, even now with all it's warts.