I have the Sigma 10-20mm f/4-5.6. It's a great lens-- decently sharp. The geometry distortion bugs me sometimes so I end up fixing it in post.
I think the Tokina 11-16 f/2.8 is better though. Actually, I have no need for a wide angle to have any zoom function. A 10mm prime would probably deliver better image quality and would be less expensive to buy. With my Sigma, most of the shots are at 10mm anyway.
Because I use the wide angle lens for mostly landscape I usually shoot at around f/11 or f/16. At f/4 the bokeh is very ugly in my opinion but I suppose it's not the way a wide angle is supposed to be used.