I got this lens (Sigma 17-70mm f/2.8-4.5) few years ago, the same week it was released for my Canon 20D and it was on my camera for many years as my walkaround lens. But there are times I needed a constant f/2.8 for low light shooting, the f/2.8 on this lens I think goes up to only 20-22mm and after that it was free fall aperture range. So, I bought a Tamron 17.50mm f/2.8 as soon as it was released because of the constant f/2.8 but then I wish the Tamron was with a longer reach of 70mm like Sigma, I missed this on the focal range on Tamron, but then I moved the Sigma 17-70mm to my Canon XTi/400D which is my backup camera because I cannot bring myself up to selling it.
But all in all I still love the Sigma. Nice color, sharp and everyhting. I highly recommend this lens.