This lens will not be competing in the same league as the Canon 28-300mmL in terms of build quality, weather seal, focusing speed and size. I will be a consumer lens like the one found in Nikon that is a fantastic lens. I will wait for reviews to then take a decision but, it's an attractive do-it-all lens for canon users. I don't think it will be the same for nikon users since they already have it and it's priced at the same range.
I will wait for the reviews. Out of all the Canon lenses, I decided to skip the Canon 28-300mm mostly because of size and price. I waited for the really good Canon 70-300mm L. I know that it is not the same but I've been very happy with this 70-300mm. In fact, Nikon would do well to try to compete with it. However I do have the Nikon 28-300 and my copy has exceeded my expectations. I really don't see buying a Tamron since I've always been let down on their lenses. I wish this new lens really competes with the Sigma Art line which I have been so happy with. I'd love to see the IQ the same as the Art line. I was testing the Tamron 150-600 at 600mm all weekend and the center quality was minimally ok but all the edges away from the center were just poor. At first I thought it was me so I tried the Wimberly, RRS RH50, and an Arcatech. I tried hand one top of lens to stabilize it, then no hand. With VC, no VC. Full sun, then shade. Rock, grass, cars, and even people (in wet suits). What I found was close subjects seemed ok but distant subjects not. I tried four different days. I'm just not in love with Tamron.