http://www.nikonchat.com/nikkor-24-70-lens-2/

Questions about the sigma 24-70 f2.8 hsm?
I have been working with a d3000 and an 18-55 kit lens for over a year now, and I am looking to replace my kit lens with the sigma 24-70 f2.8 hsm, because I’m too cheap to afford the nikkor version. I have a few questions about the sigma:
1) It doesn’t have image stabilization. Does the speed of the lens make up for it?
2) Obviously it isn’t as great as the nikon 24-70 f2.8, but is it still pretty sharp, or at least a lot sharper than my 18-55 kit lens?
3) Is the range of the lens long enough for event photography?
4) Is it durable?
5) Is f2.8 sufficient enough with throwing the background out of focus for portraits?
1) To some degree. The stabilization helps reduce camera shake, allowing you to shoot at slower shutter speeds, but does nothing for moving objects. The faster lens will allow a faster shutter speed, reducing the effect of camera shake and reducing the blur on moving objects. However, I don’t recommend shooting this lens wide open (See response to #2).
2) It’s sharper than your 18-55 — at f/4 or more. My copy was soft when shooting wide open at f/2.8
3) Depends on the venue and your shooting style; if you’re constrained to shooting from a distance, 70mm may not be close enough; conversely, if you prefer/need a wide angle, with a crop body like the D3000, 24mm may not be wide enough
4) Yes. It’s a heavy, sturdy-feeling lens; it;s not as rugged as the Nikon, but the build quality is not going to be a problem.
5) Yes; more so at the 70mm end.
Nikon 24-70 focusing noise