Are Third Party Camera Lenses Worth Buying?

When people buy a Canon or Nikon camera, they often assume that they can only buy Canon or Nikon lenses. But that isn’t true. While Nikon lenses won’t work on your Canon camera, there are third-party lens manufacturers—such as Sigma, Tamron, Tokina, Samyang (also sold as Rokinon), Opteka, Yongnuo and Zeiss—that make lenses for Canon, Nikon, and sometimes Sony and other cameras. But are they any good?
by Harry Guinness via How-To Geek