I'm thinking of taking the chance on a Sigma 120-400. I currently own the 17-50, and before that, the 17-70. The 17-50 is a truly awesome, "L Quality" lens. Certainly the new f/1.4, is of the highest caliber. If I had a full frame camera, and sold a lot of other stuff (which I might)...I would get that lens also.
The main issue people have had with Sigma, myself included, was AF. I have no doubt their new line will probably never suffer those issues. And they certainly know how to design glass that meets a price point, and delivers quality beyond it.
I really am itching to try the new 120-300 f/2.8 with newly designed elements including two fluorite. Nobody knows when it will get released, it could be 40 years from now, or a thousand...or next week.