Jim, so far so good. I haven't been out on a trip with it yet though. I'm going to replace my Tamron 150-600 with it (1.4x ordered) 3X as much $, but you only live once! It focuses much faster and accurately than the Tamron, so will be much better for birding, and is light enough that I'll carry it along with the 24-105mm as a 2 lens kit on many hike/climbs.