Do not buy! Crash! Crash! Crash! Crash! Crash! Reviews are based on the game, not the 'actual' stability of the game!
For most people, G&K fixed many of the critical issues in the
Civ V franchise and the game was more or less playable again for the majority, then comes along "Brave New World" and completely ruins that stability, it is pretty common knowledge that the original had some pretty nasty memory leaks and the second expansion seemed to fix those issues, however BNW has either broken those fixes, or created a whole new issues. Given Firaxis track record, honestly ~ I don't think these problems are going to be fixed anytime soon, especially for nvidia owners who often see appcrashes with generic "0xC0000005" errors - just google yourself to see all the angry players, or check out the steam forums.
Do yourself a favor - avoid BNW until the majority of the critical issues are fixed, save your money for something that is actually stable and playable. BNW multiplayer mode is all but impossible to play.
That being said ~ IF, and that is a huge IF ~ the actual stability of the Civ V series was fixed and never an issue, then clearly it would have deserved such high praise. But unfortunately reviewers, both professional and amateur never bring into account the actual overall stability of games into the reviews, and honestly ~ they should.