Until now, I've been using an old Canon EF 75-300 f/4-5.6 II (non-USM) lens on my new 6D body. I've got my eye on a new super-telephoto zoom and I'll most likely end up with a Canon 100-400 very shortly. I'd really like to try the new Tamron 150-600 but it just has not been available. Besides, I think I'm going to want the faster AF the Canon offers, even at the loss of reach. I've read that a Kenko 1.4X PRO extender will still allow AF to work on the 100-400, so that could be an option for regaining the extra reach.
Today, I captured a Baltimore Oriole - a new sighting for me. I also got a feisty Red-wing Blackbird, a Blue Jay, and Song Sparrow. The following images are jpegs straight from the camera - no editing whatsoever. (Check my flicker stream for more refined images.)
Red-winged Blackbird - he's got his lunch!