Are you looking for third party lens alternatives? Take a look at this selection by DPReview.com:
The Sigma 18-35 mm F1.8 DC HSM A lens has some impressive sharpness score, even beating the Canon EF 24-70mm F2.8L II USM!
The only things lacking is the focal range and stabilization. See the article from DxOMark below:
Sigma’s new 18-35mm f1.8 zoom lens looks good on specification and on price too! Certainly a lens to watch.
Sigma’s Game-Changing 18-35mm f/1.8 Zoom Lens to Cost Just $799
As an industry first, Sigma has planned to release a USB Dock for customizing their lens