The only fault I have found with the Tamron is that numerous people are having problems with AF in AI Servo mode. It sounds like their copies are defective.
Huh ... Numerous users having problems with AF would mean that the problems with the AF unit of the production lenses are to be expected.
To suggest that there are numerous defective lenses out in the market speaks so much for the confidence you have with Tamron.
Don't forget something.. like Sigma, the Tamron brand is definitely changing their name, investing big money like: hiring new engineers, designers, professional's and buying new materials (fine glasses, UD elements, fluorite similar's, better plastics, etc)..
Now the old Tamron's are like the old Sigmas... bad lenses using cheap electronics and parts.. problems with focusing, cheap glasses resulting soft images, OOF, noise AF, etc...
But now, things have changed, look the New Sigma lenses (35mm 1.4 ART, 120-300 2.8 S OS, 50mm 1.4 ART, 18-35 1.8 ), like the new Tamron's (70-200 2.8 VC, 24-70 2.8 VC, 150-600 VC, 90 macro 2.8 VC) ... all amazing piece s of glass competing with the top notch from Canon and Nikon.. for half the price, just amazing!
So, the new 150-600 VC, belongs to the new Tamron line.. which means, high quality stuff!