Gets pretty good sharpness when stopped down. Vignetting issues, but resolved when stopped down. CA very minimal. Center sharpness is very good but not fantastic. IS works fine, and I've had no issues shooting wide open with this lens. Sharpness is still acceptable. Of course it can't be compared to the 70-200 F/4L wide open. Key thing about this lens is it's compactness and weight. Build quality is also good. Non metal mount, but I don't think this lens needs a metal mount. It's very well-built. Focusing speed is quick and relatively silent despite lack of USM. It hunts just a little bit in low light with my 400D. If you've got a 40D it might be a little better though.
Good for someone on a budget looking for an IS telephoto that can produce some good images. It's also a great travel lens. Pack this in with Tamron 17-50 f/2.8 and you've got quite an affordable combination. Overall rating? Decent. Not fantastic, but decent.
These people have never used an L lens then. But anyway I feel most people who comment on lenses don't know sh*t about them. I know too many people who shoot L lenses, Zeiss lenses, and top bodies like the D300 and yet produce mediocre crap on them. They are more like lens traders than photographers.......
I hold much, much more respect for people who make the most of what they have and continually improve their photography with what little they have... so that when they finally get the L lens they know exactly that it's what they need and want and proceed to make great images with them.
Sorry for ranting. I'm sure some of you share the same sentiments anyway.
