Yes, for the most part, the advantage is price. Like most 'premium' brands, you end up paying extra for Canon or Nikon lenses.
When comparing lenses, you can't really compare the whole brand against another, you have to do it on a lens by lens basis. Canon has some pretty terrible lenses and some fantastic lenses. Sigma/Tamron/Tokina have some gems and some dogs as well.
I've used Canon, Sigma, Vivitar and Tamron lenses on my Canon cameras. I would love to have all my lenses be top of the line Canon 'L' lenses, but that is a very expensive goal.