I rented the Canon 100-400L IS 5.6ish and I liked it. Not as sharp as I'd like it to be, but at 220-350, imagine quality is easily > than a 70-200 2.8 L IS MK II with a 1.4 extender (i can imagine with 2x). Will be posting pictures soon, but I'm at work and I'm basing this on 100% camera screen zooms. If my testing keeps this up, I will be returning the extender. Just my two cents for everyone.