I'm very happy with both my Tamron 17-50 and 100-400. They are both great, sharp lenses. I can't speak to all of the Tamrons out there, but the notion that Tamron only makes second class lenses is silly.
I have a love/hate affair with my G1 Tammy 150-600. I've had the typical AF freeze, and had to send it in for repairs when it got really bad at one point. On the other hand, 99.5 % of the time I get quick, accurate autofocus and very good IQ. If I had a big old bag of money, I'd probably grab the G2 version or possibly the Nikon 200-500, but for now it's better the devil I know than the one I don't.