Sorry to completely not answer your question, but...
Yes, it is a lens I've looked at quite a bit, but that's not why I'm replying; Yesterday marked the end of a customer service story that I'm afraid Canon would have trouble matching.
To make a long story short, yesterday morning I received from Sigma (at no charge, except the expedited shipping I'd requested) a brand new 24-70mm f2.8 to replace one I'd bought used on ebay two years ago which had had persistent AF problems since I switched to a 20D last year.
No warranty, no paperwork, no service charge, no questions asked. They couldn't fix it, so they gave me a new one.
Once upon another lifetime ago, I spent several years working retail, where I observed that ALL brands have problems, even lemons - and the safest place to put your money is with the people who take responsibility for their mistakes instead of insisting they don't make any.
If Canon could compete on a customer service level, it would almost be a no-brainer; after all, this focal length and f-stop is one place where the two companies are closest in price. But I'm rambling. All I wanted to say is Sigma has taken good care of me. Canon's been fair, so far, but I've also been lucky not to have to ask for any big favors from them.