I think TomScott has a good view. Canon lenses, at least the L series, seem to be built more sturdy than the Sigma and Tamron equivalents. Of course that also comes with larger size and greater weight, as well as higher price.
I view the L series as good tools for pros who are rough on their equipment and can write off the price on their tax return. They are probably signed up with Canon Professional Service and can get repairs or loaners quickly.
Me, I'm only semi-pro and I treat my lenses very gently. If a non-Canon lens has something I want and gets good reviews, I'll buy it.
I had a Canon EF-S 17-55 IS with a loose zoom assembly. It took Canon Service three tries to figure it out. I sold that puppy then and got a Sigma 17-50 IS that works fine and is just as sharp on my 7D.
When I was using a Canon 5D classic w/o MFA, I went crazy trying to find lenses that would focus well with it. I bought the Sigma 50 f/1.4 and was not very impressed, because of the poor autofocus performance. I even tried renting the famous 24-70 L from Lens Rentals.com - what a disaster - the focus was all over the place. I think it was rented too many times by professionals, LOL!
I finally got lucky and found a Tamron 28-75 on Craigslist for about $260. I was amazed to find that it worked perfectly and was tack sharp with that old 5D classic. I am still using it today on my 5D3. Who needs an L lens?
And that Sigma 50 1.4? It focuses beautifully on the 5D3 now. Autofocus performance depends as much on the camera as on the lens.
I don't worry too much about the brand any more. Tomorrow I will receive my first L lens, a 135 f/2.0 L that was on sale for $899. It will be an outdoor portrait lens for my 5d3. I am prepared to be impressed, but my record with lenses has not been very good. Wish me luck!