It's true that the Canon 24-105 F4 L is not really impressive. One of the "cheapest" L.. but the range is very useful when we want to walk "light".
That's said, the Sigma is much bigger and heavier, and, I think, every Canon 24-105 owner has a better lens when he wants to really take a very good shot on a specific "goal" subject. I'm not sure if I would change if I was in that case.
And, my wife and I have had already 2 sigma, one small for the first reflex of my wife.. I remember it was something like 24-135.. I'm not sure.. the first example we receive was just out of focus. The second, even well focused, was not sharp at all.
Then, more recently, we bought the 100-300 f2.8 is os hsm (first stabilized version), and after 4 months, we had it back to service.. the stabilizer was broken and making noise. they changed it and it is now ok.
But, 2 sigma lenses = 2 lenses back to sigma.
And, 4 canon lenses, 4 lenses without any problems.
I'm not sure I would buy a new sigma lens...