I've been reading every review I could find on the 70-200L f/4, basically to justify the expense of this pro-lens.
To finally prove my point, and of course convince my wife... I took my 20D to my pro-dealer, and tested the EF 75-300mm F4-5.6 USM against the 70-200L IS F4.
I performed a very quick and dirty test: both lenses at the widest zoom (75 vs 70mm), middle of the range (135mm) and finally at 200mm. (I did also try the 300mm to see how it compares to the 200mm setting).
As there was no quick solution to a tripod, I simply rested the camera on a display stand in the store, and shot towards the window. This would give me enough strong backlight, I hoped, to cause some serious CA. Camera on Av, highest JPG res. Like I said - quick 'n dirty.
Back at my desk, opened the images in PS CS2, and was admiring the quality of the "L" lens images. Or wait - that doesn't make sense... these are the EF results?!?!?! Check the exif data; darn. THESE are the "EF" and THOSE are the "L" results. Admittedly, one ONE shot I can see the lower right corner being superior from the L lens, but seriously folks - I can no longer justify the higher cost of the "L".
I'd be glad to post the six un-altered, original images somewhere if someone has the space - the images are about 2.5MB each.
See, and weep. (If you bought the "L", that is). Perhaps this was a fluke - a darn good EF copy. Perhaps not.