At first I was very happy with this lense, it provided the range I needed and seemed destined to be a great walk-around lense. Price for performance, on paper, was best-in-class.
After several months of usage however, I came to the conclusion that the sweet spots I was expecting never quite arrived. Very soft at the wide angle, the midrange was average performance but not exceedingly crisp, and once you get to the upper third of the zoom, autofocus becomes a challenge. The knock on this lense is that it has a tendency to backfocus, which I have also experienced consistently.
The size is decent, the 'feel' is not as cheap as you'd expect. The motor is noisy, but not excessively so -- as long as you're not expecting USM you'll be fine.
For me this lense was a compromise between some higher end L or I/S lenses; in the end the $200-400 I saved was a false economy since I'm ultimately disappointed with the trade off. At this point my $100 50mm f1.8 EF II prime sits on my camera 90% of the time until I can replace the 18-125.
Whether Q/A failed and I have a bad lense, or this is the expected overall performance I cannot say. I would not buy this lense again, and if pressed would say that my original 18-50mm 3.5-5.6 DG Sigma performed better in the range it covered.