Ive only flown true BE once (i say true because i dont count ATL-STT's BE as real BE) from MSP-LHR, got an op-up as a GM, i was flying with a friend and half the BE cabin was empty and he wasnt upgraded with me, while asking about it i got yelled at by the FA because "all these people paid good money for the seats and you should be happy you got one free" after she walked away 3 people around me turned to me, 1 was bumped from his UA coach (i think it was) flight and got put in BE on DL, 2 were other medallions (a GM and PM) who were op-ups also.

Tho thats just my experience id say about 1/3-2/3 are paid BE, rest are non-rev, op-up, SWUs, etc.