Although a friend of mine tried it out as a rental along with an R5. He was really wowed at first but after using it for a bit he wasn't as impressed with the results. Still, for a birder, at $1K coupled with the R5's eye autofocus it doesn't have any real competitors. Not everyone needs or can pay for the higher end and very heavy lenses. (And, with modern full frame digital cameras, the smaller fixed aperature doesn't matter nearly as much.)