In the meantime, I was also lucky enough to get in three weeks of time with the full-frame 5D Mark II, on which I had the 28mm lens most of the time. It's a vastly different lens on the two cameras. Not because the 40D's a dog compared to the shiny 5Dmk2...far from it. It's just that it's a wide-angle lens on the new camera, and a normal lens on the 40D.
Performance-wise, I noticed no difference in the 28mm on either camera. It uses Canon's ultrasonic focus, of which I am a huge fan, and locks focus almost instantly. I'd compare it to the 85mm f/1.8 and 50mm f/1.4 lenses.
I didn't do any pixel-peeping on the corners of 28mm files, nor did I conduct any tripod tests. I did notice some vignetting when I used it on the 5D Mark II, but it tended to be under extreme conditions (and is easily dealt with in Lightroom). All I know is this: it's my favorite lens in my collection, and I'd say that it's well worth its $420 price tag.
Here's why I like it:
- build quality on par with Canon 85/1.8
- super-fast focus
- focuses really close
- full-time manual focus override
- very sharp starting at f/2.8 or so (not tested, just my impression)
- nice bokeh
- 7 aperture blades make nice sunstars when stopped down
- small and lightweight, makes a wonderful walk-around rig
- no hood included
A similar lens is Sigma's 30mm f/1.4. A buddy of mine has that lens and loves it - it's on his camera most of the time, focuses fast, and is 2/3-stop faster than the 28/1.8. But it only covers crop sensors, so for me the choice was easy.
It fits all the criteria I set down for it: pictures look great wide open, it focuses faster than I can shoot, it's small and light. At f/1.8 pictures are slightly soft, but since when does a picture have to be sharp to be good?