I just got my 5D back from getting a tune up, so I thought I'd do some comparison shooting in the backyard. I got out my Tamron 28-75 f/2.8 and my Canon L 24-70 f/2.8, and picked a scene with lots of very fine detail. I was mainly interested in how they did at f/2.8 at the long end of the focal length range. I shot a series of pics, and then got down to some serious pixel peeping. I compared them with my normal sharpening, and then I compared them over-sharpened. The first thing I noticed is that with the same ISO, shutter, and aperture the Canon L was at least 1/3rd stop darker. The second thing I noticed is that both lenses were razor sharp at f/2.8. Once I adjusted the exposure to match I could not see any difference at 100% magnification (20x30+ print size). I had to increase magnification to 400%, and seriously oversharpen/clarify to see any difference. Pushed to extremes the Canon appeared to have a slight edge, but it was still so close that I wondered if it were my imagination. The differences were so slight that there's no point in even posting the photos. The Canon L's AF is much faster, but while pokey in comparison I've found the Tamron to be just as accurate. My subjects never move fast enough for it to matter. Anyway, for those who are worried that there must be a big difference in image quality for the $900+ price tag difference I can assure you there isn't. It does appear that the Tamron is slightly faster even though f/2.8 should be the same for both.