We have looked the recent Sigma 100-400mm f/5-f/6.3 DG OS HSM C lens and found it to be a very capable lens. Tamron could not sit aside and watch Sigma take all trophies and so it also released a version of this lens which appears to do a very good job as well.
The build quality seems a bit better than the Sigma and just a bit longer. Frost claims that the stabilization on the Tamron is better than the Sigma. The focus ring is nice as well and the AF is fast the silent (at 400mm is a little slower but still accurate).
The lens is sharp in the middle wide open at all focal lengths – the corners are a little soft so you will need to close down the aperture to around f/8 or f/11).
As for pricing, the Tamron 100-400mm f/4.5-6.3 VC USD sells for around $800 which is significantly less than the comparable Canon and Nikon lenses.