OK, I've had mine for a week and have taken a few "bellweather" shots with it (at least for my purposes).
- Focusing is just fine, even for kids. Seems fast enough. I also have a 17-40 F4L and a 70-200 F4L, so I've seen REALLY fast AF - and this ain't it - but I haven't missed a shot of my kids yet due to slow AF. Good enough
- Image quality. This is a big subjective area. I'll elaborate below
- Small size and weight. Works well with my Rebel XT. Not as small/light as the kit lens, but that's understandable
- The small max apertures have me struggling with ISO and steady hands at anything above about 135mm. Cries out for a flash (and requires one indoors)
- Generally soft shots at almost all apertures
- The extending barrel feels fragile and takes up space
- The zoom lock switch is one more thing to remember before using the camera
- No Image Stabilization!
I took off my 17-40 and put this thing on... and realized I had to learn to use this lens more than any other I'd had so far. The compromises it presents mean more work for the photographer. I have been spoiled by constant F/4 lenses....
I have concluded the following:
- It needs higher ISO, i.e. 400 or above, outdoors, without flash, unless in bright sunlight (unless you want to shoot wide open - good luck!)
- Even stopped down, requires more sharpening than my L lenses, and maybe a touch more contrast/saturation adjustment
With this "vacation" lens, the decisions Canon made with the Rebel XT body all start to make sense. For many consumers, a lens like this may be the only one they have. Consequently, the standard settings for contrast/saturation/sharpening all of a sudden make sense (I toned those down with my L glass, or really shot RAW most of the time).
The image quality (given the above adjustments to JPEGs) is ultimately adequate for the purpose. Now that I've taken a few shots with it that I've scrutinized, I'd say for fine art bigger than 8x10 or maybe 11x14, this is not the best lens - it's just not sharp enough to give that kind of detail. In a pinch it'll do, so it's perfect for a hobbyist to take on a family trip and not be stuck with a P+S... i.e. break out the tripod, flip it to RAW, stop it down to F/8 or F/11, and get a "decent" shot. But the results won't touch what the L lenses do at least in terms of sharpness and CA. I've got those lenses too, and I can generally tell which shots came from this lens vs. the others.
That said, I'm happy with the purchase. It seems people are having problems with the Sigma 18-200 OS, which is the one I originally wanted... this lens with IS would be a nice thing (if they maintained the small size/weight.
I'm happy with mine - and for those "prosumers" who are thinking of getting it. go for it, it's well matched to the Rebel bodies and can give pretty good shots. Just be prepared to manage the camera shake (especially at 250mm, F/8 for decent image quality.... you need ISO 400, or even 800).