I have used Sigma, Tamron as well as Canon branded lenses for almost 30 years.
My current set up is a Canon eos5d Mark 2 and 7D, with these cameras I have a Canon 'L' Series EF24-105mm, EF70-300 IS USM, EF50mm f1.8, Sigma 12-24mm and Tamron 90mm macro. I bought these because they each come highly commended by various respectable monthly magazines and of course online forums.
I note that Sigma have recently launched a number of lenses that are leading the field just now, eg; 180mm f2.8 macro, 30mm F1.4 DC HSM, 18-35mm F1.8 DC HSM, 24-70mm F2.8 IF EX DG HSM.
Tamron have their highly acclaimed 90mm macro and SP 17-50mm F/2.8 VC.
The independent lens manufacturers can hold their heads up very high with the offerings they bestow upon us as a quality and cost effective alternative.
When I do decide to purchase another lens I will do what I have always done, research all alternatives and if possible visit shops that stock the lenses and try them on my camera for feel, balance and operation. If possible borrow the lens from a friend.
You will always have people (regardless of hobby) who want to be seen to have the best of everything and photography is no exception, but it won't make them great photographers.