The pictures taken by Isaac shows how the Tamron is supposed to deliver. I'm pretty impressed how sharp and well focused they all are. I must say they all are taken in good light and thats mean a lot for the quality. So Isaac, you know when to take the pictures and when not to do it. I have a Sigma 150-500 and was considering to exchange it with the Tamrom, but I will hold it a bit more because I belive it's also have something to do with the person behind the camera. I have seen a lot of bad pictures taken with the Tamron, both in test and at Flickr, but maybe they are taken in bad light or by a not so experienced person. It's the same with the Sigma 150-500, I have seen a lot of bad pictures, but also a lot of pictures which almost compare to the Tamron. But keep on giving us inspiration how it's done, Isaac.
I have found that there are many components to taking sharp pics. I will list them in order of what I think are the most important to me.
Light. The most important for this and any other lens that I have. I do not own a full frame camera and rarely push the ISO over 800. I find that when I did with my 20d, 70d and now the 7d2 that the pics suffer. I am not saying that others can not get real quality shots with high ISO, but I certainly struggle with that. Most times when the light is bad and I am birding I do not even take the camera. Or I only bring it to document a rarity and not for the purposes of getting pics.
A close subject. In every circumstance, the best pics I have gotten are pics where the subjects are close and there is no need for a heavy crop.
Good field craft. Staying still and/or approaching wildlife cautiously to allow for close camera work.
Holding camera steady and proper settings.
Post production. Only so much that I can do in post. If I messed up in the field, then I just delete the pic.
Then there are things that I am still learning and working on. Composition, artistic shots, etc. that separate a sharp, in focus, well lit shot from a true beauty.
I do not own nor have I ever tried the Sigma 150-500. From all accounts I have read the Tamron is sharper. That seems to be a consistent theme even from people that went from the Sigma to the Tamron. If you find yourself wishing for extra reach, it is a very good option.