Can someone please explain to me what the attraction of this lens is? I look at the 70 - 200mm f/2.8 IS L (Â£1750) and the 200mm f/2.8 L (Â£615) and I can see why someone would buy but when that is upscaled to 400mm we have the 100 - 400mm f/4.5 - 5.6 IS L (Â£1100) compared to the current 400mm f/5.6 L (Â£1065) there's no saving at all and no IS, there's no gain in speed and you lose the zoom option, even if IS is added and an optical upgrade the price will rise far above its current level probably closer to Â£1500, what is the attraction in that?
Well, I think this is one of lens with the highest Cost/performance value among Canon's lineup. While it lacks the flexibility of the 100-400 and the IS, in exchange you get the advantages of prime over zooms - faster focus and sharper images. Another big issue for people is price (in Taiwan, the price of 100-400 [roughly 46,000~56,000 NT] is a bit higher than 400 [roughly 38,000~41,000 NT]).
Adding the IS to the 400 prime would be cool, but if the price sees a significant increase, I'd wager there's going to be a rush to pick up the old generation 400mm.
Of couse, a 500mm would be nice, but that's a totally different field (and probably bigger and more expensive)