I've got one and I love it. I use it with a 1.6 crop body. Tack sharp wide open, fast autofocus and not very heavy. I wish it had IS, because I don't ever seem to use it in light that would allow handheld shots.
Kenko 1.4 DG tele is practically glued to this lens (with taped pins). Autofocus on mine works great, very rarely hunts with the tc. But if you have 40D then you might get problems with the center af point.
I noticed that the chromatic aberations are more visible with the TC - purple fringing, but it's nothing to worry about. Sharpness is almost the same as without TC, so I don't bother stopping down unless I need extra dof.
As far as faults go:
- no IS
- minimum focusing distance 3.5m
- only 400mm and only 5.6
)
Overall a great lens.