Recently something changed, though. First zoom lenses for APS-C/DX sensors which were really fast aperture wise were produced by Samsung (the 16-50 mm f/2.0-2.8) and Sigma (the 18-35 mm f/1.8).
solid casing; unique parameters; brilliant image quality in the frame centre no matter what focal length you employ; very good image quality on the edge of the APS-C/DX sensor; imperceptible longitudinal chromatic aberration; moderate distortion; slight coma; low astigmatism; slight vignetting level...
bad technical solution concerning the AF/MF ring; very weak performance against bright light