I had a 100-400 at the same time as 70-200 2.8 L IS II + Extender II and did a direct comparison at 400mm. Yes, sharpness takes a hit, but considering the cost, weight, and space required to keep that lens around just for additional sharpness in the 200-400 range, I decided to sell it. I asked myself, "Am I more likely to bring one lens and through the extender in my bag, or schlep two lenses?" The answer depends on what is being shot. I only took the 100-400 to the airshow. I brought the 70-200 + extender to a pro motocross race....most of the time, the extender stayed off, but I did get some great shots with it on. I am at work now, but can update with images tonight. In the end I sold the 100-400. Don't get me wrong, it's a fantastic lens.