I have owned a 24-105L before as well as 2 copies of the 24-70 f2.8L II. Sold all for financial reasons ... got the Sigma last week. It's not amazing but pretty darn good. Great for the price.
24mm Distortion and corner sharpness are better than the Canon 24-105L. The wide end is where I mostly use my 24 zooms.
OS is not bad either. Shot this in the evening, on a moving and bobbling Speedboat yesterday.
This has been gently PP'ed but I have not MA'd the lens yet... it needs a little maybe but I have not done scientific tests.
Look and feel is solid, barrel is nice and snug, feels more robust than the canon 25-105. Great finishing.
The Canon 24-105 is selling for $600-650 new, do you feel the Sigma is worth the 40-50% price premium?