I have it, and love it. I really cant find any negatives about it. I must admit i don't use it much as my 24-70 f/2.8 mkii covers this FL, and does it slightly better, but i have no hesitation to use it as a very lightweight combo with my 80D or 650D.
I have this and the 40mm f/2.8 and while the 24mm is more useful i prefer the 40mm for its longer reach.
You see I have the opposite problem, I have the 50mm 1.8 STM which I adore , but it's too close for most of what I do.
In theory the 18-50 2.8 is ideal ... but in practice I'm just not getting on with it , I need something fast and this just isn't , I even thought it was faulty because It was so slow compared to my other lenses
but it's not faulty... just takes its time lol , I can get a really good picture out of it , it turns out , but speed is not it's strong point.
So the 24mm is sounding like a good option for me at the moment
I took the 18-50 sigma to a big horse show , and when I could stand outside in good light , and prefocus a spot where I knew my subject was going to be (Like with showjumping courses) It got me some really nice , pin sharp photos.
But when I am at the beach trying to get photos of the dogs running around, there's no chance , it just can't focus quick enough , in that instance my 18-55 STM beats it hands down in terms of focus speed.