Send it in to sigma for cleaning. I had good experiences with their customer service.You'll get a cleaned calibrated updated lens back at a decent price. I sent in a 50 for focus issues. Not that long ago I sent canon a brand new 60 macro b/c it had a shiny piece of glass behind the front.e. So it can happen to anyone.. Good luck.
Most of the time it depends on the shot. I prefer softboxes indoors/in studio as they are easier to setup and control unlike in outdoors especially in windy conditions. Indoors I can also use a tripod more conveniently.
Outdoors - mostly for fill in I use flash with a diffuser but none of these are written in stone so I improvise according to what the cutomer/I/the shot wants
Before I bought it I read a lot about this lens not being sharp enough... Maybe I was just lucky with my copy. Certainly it's good enough for me and it quickly became my favorite all around lens. (I shot these on a 60D and even though I can't MA my lenses on it like on my 7D or 5D2 somehow I never really needed to.. )
My site is hosted at zenfolio and they collaborate with Mpix, so I usually order from them. So far I had no problem with them. (neither my customers) Plus if I ever will they have some pretty good satisfaction guarantees.