I bought BGN:
Pentax M42 SMC lenses: badassly clean and smooth. real jewels that I actually sold for the same amount later when I had to.
Nikon FM2n: lots of scratch on the top plate, film door that is a bit temperamental to close (just a bit), and 1/4000s shutter speed about half a stop slow. Nevertheless it has never choked.
Nikon AI lenses: nothing wrong here, they just feel like they've been used, but the glass is pristine and the results are up to par.
Rolleiflex T: the m******r just broke in my hands, and I am returning it.
Morality of the story: anything goes. That's why they have a return policy that they honour.
This time I'm buying a Mamiya C330 with 105mm DS in EX condition. For small items, I think BGN grade is a good option. But for bigger investments, I decided to stick to EX from now on. Certain cameras are more temperamental than others--the Rollei T I got was at least 40 years old, so something was bound to happen to it at some point.