Now this sounds interesting. I’ve a love-hate relationship with my IS 100-400mm f/4-5.6L lens. But up til now, there hasn’t been anything other than the 300mm f4L that comes close. If this new lens has faster autofocus (my main gripe with the 100-400) and isn’t a huge amount more than the current 100-400 price, then this could be an ideal replacement. But I fear that being f/4 throughout, it ain’t going to be around the £1000-£1500 mark… The extra length won’t go amiss for wildlife and adventure sports, but one other question remains – how heavy will it be? The 100-400 is a beast to lug up mountains, I can’t see this being lighter, but how much heavier will it be? That and the price could be the deciding factors!
Of course, being f/4 rather than f/5.6, with a 1.4x included, and longer, I really can’t see it being a direct replacement for the 100-400 – more a serious step up. Which is a shame, as a faster AF 100-400 at a similar price to current would be a fantastic offering from Canon.