Optical lens great, price / performance ratio questionable

First of all: I use this lens for just over a year at a 40D and relatively frequent (more or less my Always plan). At the imaging performance in my opinion is hardly any complaints: Good sharpness, great color reproduction, etc, that's all not an issue, but I want it no longer received, the others have made to satisfy.
Only the autofocus is with me far in my opinion should be as reliable as it, here I put the blame on the camera but whose AF is not exactly famous for great performance even in wide angle.

Is quite to discuss whether the build quality is the price justice, and there comes from me (personal opinion) a clear NO:
The lens is a "plastic bomber", the zoom ring is not in 100% firm on the housing, the focus ring has also a little game. After two-three months of normal, or even frequent use (interiors, cityscapes, at least not yet desert beach) I had already clearly visible dust particles under the front lens. After about a year now looked as if someone had since breakfast indoors. So far there are no visible effects, but once those are to make out a cleaning unfortunately be unumgängilch.
The dust problem is the way - as I do in the meantime - by attaching a (protective) fix the filter, because the dust apparently penetrates between the front lens and housing. Unfortunately, I did not know that then. Besides, it is no secret that you can affect the optical performance of the lenses massively with filters.

Conclusion: Actually a great lens with impressive image quality, but in terms of processing - very disappointed - especially at this price. The dust problem is, unfortunately, not a myth but a real fact that you can fight through the use of a filter. Lens Hood Canon is typical "cheap" on (ie non-L) lenses not included and costs about 30 EUR again.
Ultimately, everyone must enttscheiden for themselves whether it is the great image quality of the lens worth the price and the extent to which the quality of workmanship is a show-stopper here. Meanwhile, a version of their (supposedly very good) was incidentally announced 17-50 with image stabilizer, so it probably finally comes competition for this lens also from Tamron.