All aspects of this lens are examined. Also there are plenty of sample images. See more sample images here.
Canon EF 200-400mm f/4L IS USM Extender 1.4x First Review
About image quality:
In my experience, and I frankly cannot be arsed to do a pointless side by side comparison, the 200-400 produces images that are perfectly good enough for publication. I have published many images with it already, from high class A3 calendars to an advertising campaign ( all with the EXIF removed by the way ), and not once has the quality been questioned.
The Canon 200-400mm lens is the first in a generation, ground breaking, and how often have we been able to say that in the past few years. Who knows whether we will get IN-BUILT teleconverters in other Canon lenses, I am sorry but I am not party to that information! Its my view that the new Canon 200-400mm is just great, thats all I can say. In fact you know what, we should start calling it the 200-560mm lens because that is really what it is. Using the teleconverter is so seamless now that I dont even think about it, in fact most of the time I have it engaged and am using a 280mm – 560mm lens, perfect for wildlife, sports and aviation photography. Thats if you have a full-frame sensor of course, if you have a cropped sensor then you are effectively using a 300-840mm lens
Full review can be seen here.