Build quality
The Sigma 17-50mm (ad) is built very solid. It does not feel as cheap as it is.
The rubber grips on the focus and zoom ring have a great texture for using it with gloves or without.
The switches of AF and OS are a bit stiffer than the canon ones, but this is not a problem, because I use them not that often.
Over time the lens withstood a lot of rough surfaces, sand and rain without any problem.
The only scratches it got are on the lens hood and the plastic part, where the AF and OS buttons are placed.
Auto Focus
The AF is great for what I expected from a lens that is almost 13 years old.
For stills photography it is fast enough and very quiet.
If you like to take videos with a microphone, that’s mounted on the hot shoe, you will hear the clicking noise of the AF.
In contentious focusing, the AF does a great job and it is very fast and accurate.
I am not sure if it’s a problem with all of the lenses or just this one I bought, but when it’s impossible for the camera to find something to focus, like a blank wall, the lens makes a loud Beeping sound and slowly turns the AF ring. This also happens, if i turn off the camera without putting the AF-ring to infinity.
Unfortunately it is not possible to turn the AF-ring by hand, when the lens is set to AF.
Sharpness
Of course the Sigma lens is not as sharp as for example a Canon RF 24-70mm f/2.8 (ad), but also, it’s not as bad as a Tamron 16-300 f/3.5 (ad).
With an open aperture of f/2.8 you can get a decent sharpness in the center of the image. The corners are a bit soft.
But if you decrease the aperture to f/4, it gets very sharp.
On a sharpness-scale from 1-10 I would give it an 8.
Pricing
I payed 150$ for my copy of the Sigma 17-50mm f/2.8, which is great value.
On my EOS R7 it is a 28-80mm f/2.8. That’s a great focal length for most of my shoots.
Overall
If you are on a budget and have an APSC sensor, this lens is a great alternative for the very expensive canon lenses.
For 150$ this is a no-brainer.
Upgrade
If you search for something better, I would recommend you the Canon 24-105mm f/4 USM (ad) paired with a speedbooster. ~600-800$