From what I can see on TDP, there's not a lot between the Sigma and Canon's 24-105 f/4s. The Sigma is perhaps a touch better at the wide end and the Canon a bit better at the long end; nothing that I think most people would notice in real life. No doubt Bryan's full review will find that the Sigma can't focus consistently...
This is a complete total and utter myth. Sigma changed the way they design their focusing chips, systems, and protocols over a year ago. I have purchased over 13 copies of recent Sigma lenses (I usually purchase 3+ copies of a lens to get the best one) including the Sigma 35mm 18-35mm and 85mm and these have been some of the best focusing lenses I have ever seen from any manufacturer and not a single one had any issue. In fact they required less Micro Adjustment than Canon's own lenses.
There is a clear dividing line between the old sigma and the new sigma, and that started in the middle of the production of the 85mm f/1.4, in fact Sigma offers an upgraded focusing chip for the 85mm 1.4 that makes a night and day difference. Literally it turned my Sigma 85mm f/1.4 from being poor to being flawless at focusing.Sigma no-longer has focusing issues.
The myth that Sigma focuses poorly is simply not true with their newer offerings.
Regarding this lens though, it seems to be noticeably better from 24mm - 70mm and slightly worse at 105mm. I think that overall it's actually a better lens for a slightly higher price, I was somewhat underwhelmed by the sample images, but after looking at the back to back I've changed my mind.
If the bokeh is good and purple fringing is low I think it would be a good buy, it's more expensive but it's better.