I have the Sigma 150-500, 5-6.3, like it and use it for wildlife and birds, use it on a monopod often but do well hand holding too. Also use it shooting high school and college baseball. It performs well when there is enough light but find myself wishing for a faster lens at times. If I were buying today I would go for the 150-600 which was not available when I bought my lens. I like the range of the 150-500 using it at both ends often but another 100mm would be nice and would use it often too if I had it.
My thoughts, the cost of either the Tamron or Sigma 150-600 would be money well spent and used often.I'm not saying they are better than the big L lenses but they don't cost nearly as much and since my shooting is a hobby only, I find them very satisfactory and fun to shoot with.