a compromise, but a good

I had the Sigma 10-20, 4 - 5.6 for comparison and it had no chance against the Canon.
Especially in real Ultra wide angle (10 to about 14mm) the Canon is open and dimmed clearly sharper, namely at the edge (very clearly) and in the middle.
Series dispersion as an excuse, I can not accept there. I want to buy products that work the same and does not need to be adjusted only with difficulty. So much for that.

By itself, the Canon 10-22 opens great new opportunities. It is properly processed, thereby very easily.
Its autofocus is thanks USM very fast and very accurate - great!
Color brilliance and authenticity are absolutely first class (so far had nothing better) and the contrast is very good as well (in this respect, the Sigma 10-20 is not).
Yes, and why "only" 4 stars?

Because the EF-S 10-22 as measured by the price simply is not soo sharp as I would like. Here would a bit more. Especially the 550D can be a very dignified Portion resolution tolerated and it gets delivered here only conditionally.
I am all in all really pleased. The competition is either blurred or does not have for me sufficient focal length range. Therefore my clear recommendation. Only everyone should know that this part here provides no perfection.

TIP: me you do not have to buy not included sun visor necessarily the outrageously expensive and unsightly manner. From 15mm to stray light can not more effectively prevent and flares are not a problem either way (see my example pictures in the gallery).