Yep, I've used __builtin_popcountll for ORB from OpenCV (256 bit binary descriptors).
Horror story: I was once developing a TRN system for a spacecraft instrument which uses an ancient x86 processor that does not have popcnt, ended up using a look-up table instead...
It seems that look up tables are still faster when memory allows it though (http://www.dalkescientific.com/writings/diary/archive/2008/0...)