I'm surprised that there wasn't more difference on the samples posted in the review at f4. Maybe there is more obvious visual benefit at other focal lengths.
If people howl about the price difference between the 24-105 and 24-70 IS not being worth it I can't see that this would stack up for those people either when you can buy the Canon version so cheap.
I stick to what I thought when I first heard the rumour; this lens is targeted at Nikon / Sony users. However anything that keeps Canon on their toes is good for Canon users.
I agree, there wasn't a huge difference until I started pixel peeping. Corners were a big difference and that's even a highlight in Sigma's own marketing material. Without being too detail focused, the Sigma seemed to perform better at 50mm than the Canon, but I don't measure this stuff *too* much. At SRP, the Sigma is cheaper than the Canon, but we all know how to get the Canon for less these days... still, holding the Sigma mattered, it's a lovely lens even for a little bit more (but not a lot).