The Sigma 18-35mm f/1.8 DC HSM lens is not available in stores yet, but the first reviews are already posted online and they all have been very positive. The lens is priced at $799 and will be available at the end of July for all major mounts (Nikon, Canon, Sony, Pentax and Sigma).
Very detailed and favorable Sigma 18-35mm f/1.8 DC HSM lens review at Lenstip:
"You should congratulate the Sigma Corporation and their optics specialists for two reasons. Firstly for their courage – they attempted to produce an instrument no other company had tried to construct before. Such a situation a lot can be forgiven so their product didn’t have to be outstanding. Still it is obvious Sigma wasn’t contended with half measures; not only they manufactured a unique instrument but also made it optically excellent in many categories."
Sigma 18-35mm f/1.8 DC HSM lens review at SLRgear:
"This is an amazingly sharp lens, even wide open at ƒ/1.8, which is typically not the case with very wide-aperture lenses. At 18mm and ƒ/1.8, the Sigma shows very little corner softness and a good portion of the center and middle of the frame are very sharp indeed."
Sigma 18-35mm f/1.8 DC HSM lens review first impressions on Sigma's blog:
"Simply put, this is a fantastic lens. It’s sharp, fast, and offers the performance of a fast prime, with the versatility of wide-to-standard zoom range for quick recomposition. For photographers and videographers working on the APS-C platform, for weddings, documentary and more, this lens is simply brilliant."
Sigma 18-35mm f/1.8 DC HSM A lens review by Ephotozine:
"Chromatic aberrations are remarkably well controlled for a zoom lens with such a bright maximum aperture. The strongest fringing can be found towards the edges of the frame at maximum aperture at 24mm, although the level of CA is still low enough to cause very few issues here."
Additional detailed review with many sample images can be found here.