Not sure if this is helpful but here are my thoughts.
I've owned the Sigma 150 - 500mm for a few years and have shot with it on a Nikon D90, D600 & D610. Considering the price of it, I've had no major complaints. I've seen some samples online from it (like the lens quality tool at the-digital-picture.com) that make it look absolutely horrendous. I really don't think it is THAT bad and I've managed to get plenty of images I think most people would find more than acceptable. Maybe I did get a "good" copy, if such a thing exists? Obviously it helps to have decent light or a camera with good ISO performance but it still represents very good value for money in my opinion. Also, it helps if you're not a million miles away from your subject but then isn't that the case with most lenses in this category? I've always handheld it (apart from once when I sat for a week waiting for a damn otter) and I never found it too big or heavy.
Having said all that, my copy packed up on me when I attached it to a D750. Sigma said it was probably just the firmware that needed updating but it turned out to be a lot worse. £207 later and my copy now has new OS, a new AF motor, a new rear barrel and a new MR sensor. Came back to me yesterday and the OS is now much quieter than it was before. Seems daft but I might sell it on now and try and make the "step up". It's annoying me that I've spent so much money on a lens that's only worth about £400 though the repair does come with a 6 month warranty so that was a bonus. I believe the OS was prone to failing on some of these Sigma super zooms so that's something to look out for.
Anyway, I've not helped ...