I'm very interested to see how the new Sigma 24-70mm f/2.8 IF EX DG HSM
Reviews on the old, non-HSM Sigma 24-70 f/2.8 were definitely a mixed bag - not the kind of performance that someone buying a nearly-$1000 lens is looking for, but this new lens appears to have a very different set of internal optics, so it will probably need to be judged on its own. If you're searching for reviews online, make sure you're reading about the new, HSM version and not the older EX DG (non-HSM) version that cost about $400.
For current Nikon DX users (and corresponding non-full-frame Canon owners), pro-style constant aperture lens purchases haven't an easy decision. The previous options were the various APS-C sensor lenses in the 17-50mm range, which have the obvious limitation that they won't be useful if you later on switch to full frame, and the full-frame, 24-70 f/2.8 lenses from Nikon and Canon that had great optics but often ran $1600-$2000 if you wanted one with a built-in focusing motor, vibration control and other modern features. (If you have that kind of budget for a single lens, you're probably shooting full-frame already.) Not to mention they are often 50% heavier than the old-school Nikon 35-70 f/2.8 lens I commonly carried when shooting film.
My super-wide-angle zoom option, like many Nikon DX shooters, is the well-regarded 12-24mm f/4 DX . It's a great lens, and since I'm carrying it most of the time I don't really need a mid-range lens that reaches down into the 16-18mm range. If the optics on the Sigma check out, it will be an ideal match - $800-$900 for a constant aperture optic that doesn't weigh a ton and on my D300 is equivalent to a 35-100mm lens. (Unfortunately, that's a serious IF... I'm not completely negative about third-party lenses, I just know there's more variation in their performance than you'd see with a Nikon or Canon branded product. Saving 50% sometimes means you need to do a little more research.)
Still can't find a decent English-language review from a trusted source, but early reports continue to be largely positive.
There are some additional sample photo (using a Canon) here:
http://www.slashgear.com/sigma-new-24-70mm-f28-if-ex-dg-hsm-hands-on-sharpness-at-wide-open-1036964/
It's been a few months since the release, and there's still no decent reviews of this lens online. Sigma Corporation, especially now that this lens isn't sold out everywhere any more, you need to run, not walk, to some of the better photo blog sites or camera review sites and get someone to do an in-depth review of this camera! Please! It's a $900 lens (almost $1000 with NYS sales tax) and most people who might buy it would really like to know how it performs in detail before laying down a wad of cash on a third-party lens for their Canon or Nikon SLR.