considering the test photos here. Not I know they are not "official" test photos, but they are more of a real world example. One was taken with my 75-300 (non IS) lens and the other with me 100-400L. Anyway, please let me know what you think of these two shots. Ovbiously ignore the unoriginal composition. haha. And yes I know there were a few spots that are blown out. Both were taken on a tripod, f8 1/20s. The 75-300 was at 300mm and the 100-400 was at 400. I also shot one at 300 but it was blurred. Everything seemed a bit blurred, I think it was my fault and not the lens. I did trun the camera off while switching lenses and did NOT move it from it's location in any way. I honestly can not say I see much difference. and If anything the much cheaper 75-300 seems to look a bit better. Well except for the tremendous CA around the white hands, but that can be taken out pretty easy in PS.
75-300
http://www.pbase.com/image/31820294
100-400
http://www.pbase.com/image/31820312
75-300
http://www.pbase.com/image/31820294
100-400
http://www.pbase.com/image/31820312