What can I say, an ingenious lens !!!
The first photos were all razor sharp, even at full aperture and across the entire focal range. Fast focusing and a very fast-working image stabilizer.
The Bokeh Buttery soft feel and build quality are just amazing for type lenses from Sigma. However, the combination 5D Mark II incl. BG and the 24-105 quite heavy, but still is great in the hand. Concerns you have but that filters are required with 82 mm diameter.
The lens comes with lens hood, a definite advantage over the Canon.
Meanwhile, I have used the lens even when photographing weddings, perfect all I can say. Fast focus, perfect image stabilization and a brilliant image quality.
A small negative point, there is namely the vignetting at 24mm. If one tests this for fun on a bright monochrome wall can be seen very well. But it can be very good with Lightroom 5.6 change if you do not want to have this.
The property was well packaged and delivered quickly.
Whether that Sigma is better than the Canon, I can not judge because I've never used the Canon lens.
I am very pleased with the 24-105 definitely and would recommend it any friend ..