They have likely the best selection of MF backs, lenses, technical cameras and accessories in the world.
I too would highly recommend B3K. They know the equipment inside out and offer exceptional service and advice, which I find invaluable when renting equipment unfamiliar to me.
By comparison, the last time I rented from them I had also contacted the rental department of one of their large competitors to enquire about availability of an Arca Swiss, Alpa or Cambo. " Never heard of any of those makes " was the reply.