I've tried a number of 100-400 IS L's over the years and i've had mixed observations. There are soft copies and there are copies with mis focussing AF. Wide open, i've found it a little soft at the short end but very sharp at the long end. So i used stop down to f5.6 across the whole focal range and then i found that it was excellent and very sharp. It vignetted a bit and the out of focus areas were a little harsh. Very strong contrast and vibrant colours too. I did feel that it was a little shorter than the claimed 400mm, which seemed to get shorter as the focus point got closer (no suprises really....most zooms do this).
A good copy is a delight and amazingly versatile. But with every copy i've tried, I've been rather underwelmed by the AF speed. It's pretty redestrian and I'm pretty sure I could manually focus better than most copies i've tried.
Where can Canon improve this lens? Pretty much in every aspect to some degree. Yes it's handling can be improved, vignetting can be reduced (the reason for the 82mm filter threads I'm guessing), there's always room for a sharpness tweek, AF speed improvement is a must as is AF accuracy. The IS system was a revolution in it's day....but things have really moved on...just play with a 70-200\2.8 L IS II to see that. So a newer and fresher IS system would be fantastic...4 stops...very nice. I hope Canon put the tripod mount in a more sensible place. Racked out to 400mm and the tripod mount is so far from the objective lens, it can't be doing much to reduce camera vibration. The 400mm f5.6 L prime has it's tripod mount nice and central and it really works well there. I'd hope that the focal length really would be 400mm and that it wouldn't reduce significantly as the focus point get closer (the achillies heel of the new 70-300L).
Is it a lens that I'd purchase? Most probably, I'd sell my 400mm f5.6 L in a flash as long as it's results are comparible. Great IQ, fast AF speed, versatility and IS are the key points. Lets hope it ticks all these boxes...then again, Canon aren't going to make any mistakes with such a key lens in their range.