I have a few L kenses and a bunch of non L. I have one sigma and it was very poor. I had to have it repaired and now it is good. I find the focus is much better on the genuine Canon lenses, as long as they are ultrasonic. The tests usually agree with me. I find lenstip very useful. The 15-85mmIS is L lens quality without the price. My point is the autofocus in the camera is better. Canon won the pro market for the fast focusing of their lenses. This is based on the system in the camera as well as the lens. The eye controlled focus was amazing, just look where you wanted to focus and bang! It was right on. I have no idea why they no longer use that system. But the best canon cameras have had more focusing points for years, 45 points and now 61!
I have to agree if you are shooting action.. I have fewer keepers with the Sigmas. Otherwise, for still shots, I prefer my Sigmas.. Build quality is light years better, but with both Canon and Sigma, there seem to be some duds every once in awhile.
I do own a few of the cheaper Canon EF-S lenses, but they just sit in the Drawer with my old Rebel XS.
Every once in awhile I dig out the 50 F1.8. It's usually fairly sharp around 2.8 and still has nice Bokeh