Upload & Sell: Off
I don't have a problem with Tamron, but their AF consistency is not as reliable as Canon, and several of their lenses have had problems with outer AF points especially. If AF consistency, sharpness, colors and the like matter to you, the verdict is still out. I'm sure it will be a fine lens, but then, so is the Sigma. Only time will tell, but there are plenty of people who will stick with the Canon. Clearly is is longer and likely has better stabilization, and if that is your only criteria, the new lens looks a winner.
It may be the only one in the world but I have a Tamron 70-300 mm f4-5.6 that is sharp enough that I can crop from 300 mm to the 100-400's 400 mm scene and find no loss of IQ what so ever. It also has the best VC, I've owned including Canons 70-200 f4 and Canons 100-400 mm. Not one problem with reliability on the Tamron. I was already thinking of selling my 100-400 before the Tamron 150-600 mm was introduced. After seeing all the samples posted in reviews, I just sold my 100-400 for enough to pay for the Tamron and have enough left to pay for a bean bag to help stabilize the lens at the 600 mm it offers.
I honestly think Tamrons policy of using the new technology to improve quality and gain customers and customer confidence is a lot more desirable than Canon or Nikon's policy to line their pockets at customer expense, Here Here Tamron!