On the third party lens, I have bought several Sigma lenses, including 24-70mm, 50mm 1.4, etc., but returned them after trial. It's a short story, none of them focusing right.
Thanks B&H and Adorama, they allow free shipping and free return without question. I like a local camera store near me, Keeble and Shuchat at Palo Alto, they simply don't carry Sigma.
I heard it's because Sigma does not pass their qualification, and the business model is not flexible.
I also have tried several Tamron, the 28-75mm is a good one. But Tamron do not have enough lenses to take me away from Canon or Nikon.
Anyway, but Nikon/Canon vs. third party is not my point. I just thought it' a shame that Contax 645 was pushed out of the market, and Hasselblad may not be much better. And now a high quality 35mm system with the desired lenses is even more expensive than a MF.