Interesting. That is one of the more positive reviews of the Sigma that I've come across. I've been considering selling a couple of lenses to fund a 24-70 f/2.8 of some sort as my photography needs/wants are changing a little, but I'm struggling to get that enthusiastic about any of them once you factor in price.
One criticism of both the Sigma and Tamron options that I've read is that they don't do well tracking moving subjects. That points me to the Canon 24-70 f/2.8L II, as AF performance is more important to me than IS (although I've really appreciated the IS on my 24-70 f/4L IS when travelling, not to mention it's relatively light weight) ... but the Canon is still A$2600 and more where I am, which I find hard to swallow for 2.8 lens covering that focal range. Decisions.