Frank R.
Ive had the Mark II for 8 years and admittedly, the improvements here are subtle. I tried a one-to-one comparison in my living room and couldnt swear Id seen a difference. Used it tonight to shoot a rock band in a local venue--a fairly tough test--and I loved it. The photos came out looking great, sharp and crisp, despite shooting at ISO 3200 to 6400 (on a Canon 5D Mk IV). The autofocus was dead-on. . .EXCEPT when I bumped the Auto-Manual switch and turned it off. Did it a couple of times through the night. Is that a flaw of the lens or just my copy? Never had the problem with the Mk II, though I definitely have with my 24-70 and ended up taping the switch in place. I mention it only so others might watch out for it; for now Im considering it Operator Error and not blaming the lens. One of the reported improvements over the Mark II was the handling lens flare. Canon was probably thinking of sun glare but for me, its stage backlighting aimed right into my lens--and it handled it great. I dont think I got a single lens flare the whole night; nothing obtrusive, anyway. What else. . .it feels just like the Mark II. Same weight, balance, controls. Image stabilization is reportedly the same and I didnt notice a difference. You could certainly make the argument that its not worth the money to upgrade. . . but overall, Im impressed with the new lens and am glad I replaced my aging and battleworn old one.