It's a masterpiece. I'm a huge civ fan, standing at 284 hours played on my steam account, but I didn't play Gods and Kings that much. But Brave New World, oh boy. I think 50 of those hours were spent so far in BNW.
It's like they finally got the cake mix close to perfection. The balance is great, the gold starvation makes the early game a lot more interesting regarding choices, instead of one "best" opener. You can still go for early war (and some aggressive leaders still do, eg, Khan), but since the opportunity cost for a failed early war is so great, you really have to focus on it to make it happen.
The culture victory revamp is great, and the new ideologies are close to the civics from Sid Meier's Alpha Centauri, a favorite of mine. The new civs are interesting, with some unique ones such as Venice. The new Great Works, Archeology and Tourism make the mid/late game also more interesting.
This is the best civilization ever, even edging out Beyond the Sword. There are maybe some very minor balance tweaks that are needed, but it feels like nitpicking compared to say, vanilla Civ5 or the pre-patch GK. After releasing this, I am wondering how they can top this in a hypothetical Civ6.