Nikon vs Sigma and Tamron?

For years I have been advising my Nikon owner students to stick with Nikon glass and they would never go wrong. But recently I have had a chance to get my hands on both a Sigma lens and a Tamron lens and the quality seems first rate as do the images from DSLR. And the price vs a similar Nikon lens was about a third less. May I please have some opinions regarding these and other non-oem lenses vs oem Nikon? I would appreciate it.

Yes, it really depends on what lens you are comparing.

Here is a Tokina 11-16mm vs. Nikon wide angle: http://www.kenrockwell.com/...1-16mm.htm