I have the Sigma 50-500 and it's a great lens. You can get some great candid shots with it.
I've also ordered a Sigma 150mm lens and a Kenko 2 x converter (I like Prime lenses too) to test out.
I tried to add a photo I took with the Sigma so you can see what it's like but I don't know if it worked.
Ok, I got it, this first photo is a photo of a Dingo I took with the Sigma 50-500
This 2nd one is an action shot taken with the same lens.
The Sigma 50-500 is a very good lens and works well with the Sigma 1.4x converter too. Used it for many years for wildlife photography, can be used hand held. Sold it and bought the Nikon 200-400. Would not hesitiate to recommend the Sigma to anyone. I look back on it with great fondness. (Would recommend the 200-400 as well of course - it's a cracker but expensive).