I just found
these samples of the Sigma 10-20, Tamrom 11-18 and the Sigma 12-24. From looking at them the Tamron is quite soft and the two Sigma's look basically the same. The edges are soft, even at f/16. But I guess thats just the way it is.
I've also found
this too. Some stunning results. I'm really thinking that I'll get this lens.