I have used the current 120-300 f2.8 a bit, and I've been quite impressed with it. I also enjoyed using a 50-500 Bigma in my Pentax days, as long as it was stopped down, and after that a 120-400 OS in Nikon fit, which was a bit soft.
Since my move to Nikon, I have owned a succession of the Nikon Super Tele's, including the 200-400 f4 as I found the zoom pretty useful. I actually sold a non OS Sigma 300 f2.8 to part fund the 200-400, and it was a mistake in hindsight. The Sigma 300mm f2.8 lens I had was stunningly sharp, even wide open, and very very close to the Nikon version in terms of speed of focus and contrast, but not so good with the Sigma TC's.
I sold the 200-400 as it wasn't very good for wildlife at 300mm +, which kind of defeated the object a bit, but I'm very happy with my current Nikon 300mm f2.8 which is exceptional even with a 1.4TC, and also very good with the 2.0TC III, and all wide open.
I really want to see Sigma succeed at this end of the market, Nikon/Canon need competition and they are a viable alternative to folks who don't want to spend the often eye watering amount of cash the big two expect you to lay down for lenses.
I've purchased all my Sigma kit new, the Nikon's have all been second hand...