I have the Quantaray, which is a re-badged Tamron. The lens is OK, it works best in bright sunlight, once the light starts to drop off you see quite a loss in quality. It suffers pretty badly from purple fringing.
It's OK for the money, but I wouldn't buy it again, I'd actually buy a Nikon 55-200 for not much more money, or shop for a used Nikon 70-300.