I've got a Sigma 150-500 OS and use it extensively throughout the year for wildlife, planes, sports and anything else really. It's a cracking lens and is really sharp, mines very good wide open. The only problem is its weight if you're not used to carrying a big lump around. You soon get used to it though, I carry mine around all day and use it handheld for long periods with no problems. For the price I don't think there's another lens to touch it. I've used mine handheld down to 1/10sec and still got a decent shot.
The Sigma 120-400 OS is another very good lens, the wife shoots wildlife with that and gets similar sharpness and it's slightly faster than the 150-500 at the long end. The price of the 120-400 is only a bit less than the 150-500 though, so it makes more sense IMO to go for the longest reach for wildlife. I use the 150-500 on my 5D3 mostly so I find the 120-400 a bit short for birding but fine for most other shots.
There's a lot of shots on my Flickr page that were taken with the 150-500 OS if you wish to have a look. Most of the wildlife and plane shots have been taken with it.