How to pros feel about 3rd party lenses?

Wanting to know if any of you pro photographers work with third party lenses. If you do, are they the same quality as Nikon and Canon cameras lenses for each brand's cameras? If you don't use them, why?

Added (1). For example. What would be the disadvantages of buying the Sigma or Tamron 24-70mm 2.8 for Nikon versus buying the Nikon lens?

Added (2). And are the disadvantages significant enough that it would be work spending the extra thousand $$

If you meant 'generic' lenses for each brand cameras the it's okay if you're on a budget. Sometimes the lens does not give 100% compatibility to the features of the camera body. Original brand lenses is still the best.

I don't use third party lenses because I prefer to use lenses specifically designed for my camera… I know they'll be 100% compatible and I know they'll perform well. I have used third party lenses in the past, but the OEM stuff has always outperformed the former. To my mind you get what you pay for.

If you are on a tight budget though, 3rd party stuff is a better option than no lens at all!

Third party lenses are just fine. They might not have quite the same quality as the Nikon brand, but they are not going to be night and day different. The Sigma 24-70 F.2.8 is an excellent lens.

However, they are not thousands cheaper. Comparable Sigma or Tamaron lenses are only going to be a few hundred dollars cheaper at most. The 24-70 is one of the exceptions, where it is actually a lot cheaper.

It's a matter of preference. But performance wise, they should all be the same. I have a Tokina 35mm f2.8 and it outperforms my nikon 50mm f1.8. Look how sharp and close to realism this shot of mine with the Tokina lens:

I'm not a pro by the way. Just sharing my thoughts.