I take wildlife pictures, mostly birds, and my cameras are Canon. I went with several telephoto lenses before going to my first 150-600.

So 55-250, 70-200, 300 f4 IS, 400 f5.6, 100-400 II, Sigma 150-600 Contemporary were my previous lenses.
I was always looking for more reach, and more flexibility, trying not to loose quality in the process.
About a year ago Ive got my Sigma 150-600 Sport, and I think it has all that I was needing for the price I was willing to pay.
Of course a 500 or 600 f4 can be better, or even the 200-400... but that is a cost I cannot pay, specially if I can get pictures that can be sell and published with the Sigma.
As always, there are some tradeoffs. The weight is one of them, and the aperture is another. Even the autofocus perfomance is not on par with the big whites.
But for the money it cost, I believe is one of the best options you can find to do some amateur and profesional wildlife pictures.
In my case, I use intensively my gear, under harsh conditions, so the solidly construccion of this Sigma is a must, which made the Sport the best 150-600 option for me.
Ive bought a foot replacement from Wimberley (AP-608) because I needed the arca swiss compatibility and the longer feet for balance in the gimbal. It was necessary for me. It is great.

I sincerely recommend the Sigma 150-600 Sport for wildlife photography.
Ive uploaded some pictures I took so you can see what is capable off, and Im sure it can be better in the hands of experienced professionals.

Super happy at a fraction of the price of Canon telephoto lenses

First Art lens I have purchased and it is worth the additional cost.  I can see a difference in the quality of the pictures it produces.

I use my lens for wildlife photography. It is very well built and as a result rather heavy. Would recommend a mono pod when using it. The image quality is excellent even in lower light conditions.

We've been talking about getting a lens for wildlife photography for a while now. After doing some research and reading reviews we decided on the Sigma. We are not disappointed. The lens is easy to use right out of the box. It's a little heavy but not unexpected. We purchased a gimbal and it offsets the weight nicely.

Yes, the lens is heavy but for my money, you won't get better pictures for the price range. Best on a tripod but will also work with a strong arm if you like. I found the focus and quality, with good light, just fine. I'm very happy with the lens, which I purchased for nesting pictures of birds such as Eagles, Herons and and Owls. As an hobby photographer am more than satisfied.

Absolutely love the 150-600 mm lens. It was just what I needed to take wildlife photos. Great product. Wonderful service by B & H.