Dustin has completed his review of the Sigma 150-600mm f/5-6.3 OS Contemporary. In his review he compares this lens against Tamron's own 150-600mm lens.
As others have said, this lens is great value at around $1000 USD and should be on your list of budget friendly supertelephoto zoom lenses.
Sigma’s more affordable ultra-telephoto lens strikes a lot of the right chords. The Sigma 150-600mm f/5-6.3 DG OS HSM C is a handsome lens that is relatively light and hand-holdable while offering great optical performance. It gives up little to its competition (save perhaps in the autofocus department), while bettering them in some ways in the image quality. I wasn’t crazy about the manual focus ring, but I also recognize that this is rarely going to be a priority for users of this lens. While not built or weather sealed to the exacting standard set by the Sport edition, it retains a lot of its handsome looks and even most of its ability to be customized. It comes with a number of well executed accessories like a nice case, carrying strap, and rubber ring to “finish” the lens with the tripod foot removed. This is, frankly, a lot of lens for the money, and is a solid pick for those of you looking for an affordable way to get major reach. Nicely done, Sigma!