Sure, Sigma has made a name for itself over the last couple years by producing professional-quality gear at absurdly low prices. Witness, for instance, their 35mm f/1.4 DC HSM, which has been praised from just about every corner as one of the best bargains on the planet. But even so, we didn't see this coming; the pricing undercuts similar offerings from the first-party manufacturers to a surprising degree.
Nikon and Canon each make 17-55mm professional zooms intended for APS-C cameras, both with f/2.8 constant apertures. Nikon's 17-55mm f/2.8G IF-ED retails for somewhere around $1,400. Canon's 17-55mm f/2.8 IS USM is a bit more reasonable at around $1,060, but it's still 25% more expensive than Sigma's lens. And Sigma's lens, it must be noted, is a full stop and a half faster.
But the Nikon and Canon lenses have been proven to be exceptionally sharp and well-built. Surely Sigma can't compete there, right?
On the contrary, the early news from lens review sites is extremely positive. SLR Gear tested a prototype copy and called it "an incredible lens at an amazing price." They go on to praise its sharpness and vignetting performance (even at f/1.8), along with its distortion and autofocus speed. For the curious, there is also a gallery of real-world images. Color us excited.
(Note, however, that the Sigma is about 20mm shorter on the long end (30mm effective) than the Nikon and Canon counterparts, and also lacks optical stabilization.)
For the moment, the 18-35mm f/1.8 is only available for preorder in Canon, Nikon, and Sigma mounts, but there are reports that Pentax K-mount and Sony A-mount versions will follow. Pre-orders for the initial mounts are available at B&H Photo.