I put together a quick comparison of these reasonably priced alternatives to modern standard f/2.8 aperture zooms. I've had the Sigma for a long time, ever since switching to Nikon back in 2012. I've really gotten great performance out of it, but occasionally look at close crops and wonder if I could get better sharpness and contrast. I've gone through a number of alternatives, and none have bested my original Sigma, so I'm still using it. In fact, it is my most used lens, and according to my lightroom smart collections I've taken almost 7,000 photos with it. The next nearest lens, unsurprisingly, the Nikon 70-200mm VR2 with around 4,800, and behind that the Olympus 25mm f/1.8 with around 2,000.
I've compared this particular lens with a Tamron 28-105 (far worse), another Sigma 24-70 EX non-HSM (this one was newer but much softer at 70mm). I also had a Tamron 28-75 2.8, but in Canon mount so I don't have any direct comparison images. I've also done some digging through the internet archives, and have read that this Sigma performs similarly or better than the Canon 24-70mm f/2.8L (original version), which is high praise. It also performs better than the Sigma 24-70 HSM, especially at the long end. However, there is another easy to find review comparing the Sigma to the Tokina using photos of a newspaper page that points to the Tokina being much better. This is definitely not the case after my testing though, and I think that the testing performed on such an old body may have been more of an issue of phase detect AF needing calibration with their copy of the Sigma lens (at the time, AF Microadjustment was not readily available). It's worth noting that the Tokina has incredible construction quality - it's very tightly assembled, the AF/MF clutch mechanism, similar to the Sigma's, is smoother and better dampened, and the felt-lined hood is deeper. The Tokina also does not extend when zooming, and it's a little thinner and longer which personally I think looks nicer and probably makes it a little more resilient to dust and water. The Sigma zooms quite a bit towards the wide end, though it is a single cam mechanism (one barrel) and made from metal with a nice textured finish (though not as nice as the Tokina's).