Yes they do make some exceptional lenses, couldn't agree to that more. Cost wise, yes they are MUCH cheaper but considering the quality you get with the Nikkor lens, you won't go back to those third party lens given that one has a heavy pocket to spend lavishly on his hobby/passion.
If one can 'easily' afford a Nikkor, he won't go for the third party lens. From the wide range of lenses being available by Nikon and Canon, only a FEW can be better by those of these 3rd party manufacturers. Nikon and Canon yet dominates them for a reason and that is superior quality due to the fact of using superior components being used in the lens production. Yes they price it also high because of the brand name, but a Nikon would be a Nikon at the end of the day quality wise
Yes research should always be done, and if one makes a decision based on rationality, whatever he chooses won't be wrong be it Sigma or Canon
they have the 2.8 for 18-50 if i'm not wrong.