Pros: This lens allows you to get great wide angle shots with little if any distortion.
Cons: The lens could be a little faster.
I do a lot of storm photography and I am a fan of nice storm structure shots. Unfortunately I am usually right up there with the storm for the lightning and details of specific storm structure so being able to get the wide angle shots required for a complete view of a storms structure was impossible without getting miles away from the storm with my other lenses.
A few of the other chasers I know were getting these awesome wide angle storm stucture shots with their Canons and they were doing it with the Sigma 10-20mm. There was none of the extreme distortion I had seen with various fish eye lenses and a few other wide angle shots I had seen.
I purchased the Sigma 10-20mm lens for my Pentax K10D in preparation for this springs severe weather and while I have only had a few chances to try it out before the storms arrive I am impressed so far.
The lens has a very nice wide field of view that allows you to get everything in the picture from right in front of it. The only time I have noticed any distortion at all is at the widest angle of 10mm and being to close to the object. You can have a little fun with the lens if you get right on something and take the picture as then you do get a bit of distortion that can have a neat effect.
The sharpness does not seem to be a problem at all. I have seen a few complaints about sharpness and focus and the only problem I can see might be that the cameras focusing system has a bit tougher to time trying to get a wide field of view like this lens creates into perfect focus.
The lens is not a real fast lens at F:4 but I was aware of that when I purchased it so it doesn't come as any surprise. The weight hasn't bothered me at all. In my case the more the system weighs the more it seems to help me stabilize hand held shots.
The only thing I can really say that is negative about the lens is that the lens cap does not fit real tight and may fall off if it is bumped while being put in a bag or case.
The Lens comes with a nice little case that can be attached to a belt.
I don't think that you can go wrong with this lens for the money you pay regardless of which camera system you buy it for.