I don't understand why everyone's so concerned about the low price. If Tamron can make a very good 70-300 for $450 why not a 150-600 for $1070? Remember, lenses aren't ground by hand anymore. Sure, more material and time is needed, and there will be a higher QA rejection rate, which I think the price appropriately reflects.
I don't expect this to be an L-quality lens, but I'm not afraid that the 150-600 quality won't be on-par with their 70-300. But I guess we'll know in a few months.
I am more concerned about AF performance with the larger lens elements.