The Sigma 10-20/4-5.6 was one of the first lenses I bought for my D90 and I think it is a very good lens and I still enjoy using it. I mostly do landscapes, thus I use the lens stopped down and I think it is quite sharp stopped down. I know some people that had the Tokina 12-24/4 and sold it, because it easily produces flares. I never used the Tokina 12-24/4 but I think the Sigma handles flares quite well. For me this is an important factor, because with landscapes and the very wide field of view, you often have the sun in the frame or close to the borders of the frame. Also the difference between 10 mm and 12 mm is big. You won't need 10 mm all the time, but it can be a lot of fun having such a wide field of view. The Sigma 8-16 is very well regarded, but personally I like the 10-20 mm range better and the Sigma 10-20 can take filters and the Sigma 8-16 cannot and for some applications like photographing waterfalls it is important to be able to use a polarizer. There is a good review by Thom Hogan:
. His conclusion is, that the Sigma has some optical flaws, but all the other lenses in the 10-20 mm range do also have them and it is good enough and a good value for the money. I agree with that, if you can get a good sample (I think the Sigma quality control issues are a bit exaggerated, you obviously only read from people that have issues, not the people that do not have issues), you will have a lot of fun with the Sigma 10-20. For pixelpeepers , other, more expensive options might be better.