It is great to have true wide angle coverage on my 30D with its 1.6x sensor. I might try a thin filter on the front since shots with the hood protector on don't show vignetting.
It is bit softer then my Sigma 24-60 and Canon 28-135 but only when you look closely at very fine details. I have some test photos on luminousnature.com from these 3 lenses - 5 gal paint pails with lettering are in scene along with a wire covered bird feeder. I also have a macro photo taken with this lens.
Over on luminous-lanscape.com is "Focusing in The Digital Era," Gary Ferguson, has some thoughts about focusing in general.
I don't know how it compares to other Sigma 15-30, I might test an other 15-30 when I am next in NYC.
Good price (I use PowerSnipe for eBay auction sniping.) But then some of the other reviews say that buying this lens sight unseen, or at least without doing some test photos, may be risky.