Well I have the Sigma 24-70 f.2.8 EX DG (Non HSM version) and don't really have any complaints in terms of image quality. I've had the lens a number of years now and shot thousands of images with it and never really thought anything was softer than it should be or had any aberrations that shouldn't have been there etc.
My only complaint about the lens is that my version doesnt have an HSM motor which means that focusing is a bit slower than my other HSM and USM lenses and a lot noisier. Being a Canon user I can only compare it to Canon lenses and yes, sure, it doesnt have the build quality of the Canon L lens and yes the picture quality of the Canon is better (but not massively) but the Sigma costs a fraction of the price and the image quality is very high.
Put it like this, I own these lenses:
Sigma 24-70 f/2.8
Canon 70-200 f/2.8 L
Disregarding the Sigma 10-20 which does distort things as any UWA will, when I look back at my photos I honestly can't tell whether I used the Sigma 24-70 or Canon 70-200 L (other than obviously the perspective) in terms of image quality. Yeah sure if I was rich I'd buy the Canon 24-70 f.2.8 L but until I win the lottery the Sigma suits me just fine.