I have to be honest with you and say I would have bought this even if it was a little soft, because, what else is there that is wider on a 1.6x camera? Nothing! Start from there for the rating. On top of this, it is sharp.
Nothing is really wide on a 1.6x camera like the 20D so do yourself a favor and don't buy the 16-35 or the 17-40 if you haven't already bought them as your "wide" angle solution as this 10-22mm is a real wide and those other lenses might become redundant in your bag if you bought them for your wide angle solution. Not that the 17-40 doesn't take excellent pictures, but they are nowhere near this wide. This is a whole new world.
Really fun and an easy decision to make to buy this one. A harder decision for me will be the telephoto end of my collection. There are so many good ones and I am not sure which to buy, but there is just one REAL wide angle for the 20D.