Well, here it is:
I believe this will be a better option than the Tamron optically, maybe I'll buy one for myself
I wouldn't be surprised if the optics are marginally better (bigger, heavier, larger front element), but my biggest concern (as with all Sigma products) will be AF accuracy. At longer focal lengths even a little miss produces an out of focus image.
Still, options are good. If this is a better lens than the Tamron it might attract the middle tier of buyers, but the fact that it may cost twice as much means that Tamron will probably still claim the majority of amateur sales.