i own at least one good lens from each of the major third party makers (tamron, tokina, sigma). I think each makes at least one or two real good lenses, if not more. I have found that you have to find the right one, and the right model. I think quality control is a missing element in the third party stuff.

I also own the 100-300 f/4 hsm sig, it's okay too, but shot wide open it's a little soft. One thing about sigma, that i dont see in other brands, including my nikon lenses, is the colors. the 120-300 that I own adds green and I dont like that, but I live with it for it's zoom capabilites. I think nikon should make one to compliment the 200-400mm f4.

I never mate any tele converters to my third party lenses, they just dont need the help - LOL. That said, I've read where guys are getting away with it on the 500 4.5 sig.

Keep in mind that the 120-300 is big and heavy, and you does not have a rear internal filter holder! It does accept front filters if you can find them, it's a 105mm piece of glass!