You are certainly asking yourself "why he is reviewing a zoom priced roundly 360€ (new) while the purpose of this web site is lowcost4DSLR?".
Read further, you will understand...
I have already been disapointed by the kit zoom provided with my first DSRL : the Konica Minolta 18-70. Good enough for landscape (when closed) but for other purposes, the primes lenses are far better and a twenty year old zoom like the Minolta AF 35-105 old is also better.
My dream was to find a single zoom, to replace all these lenses, with a fast constant aperture, a fast autofocus and with a very good image quality.
So I purchased the Tamron 17-50 XR DI2 f/2.8 with the target for it to be my sole indoor and landscape lens.
Considering it's range and wide constant f/2.8 aperture, this zoom is compact. When mounted on the Sony Alpha 700, it is well balanced and nice to handle.
At 50mm (fully extended)
The front element (diameter 68mm) does not rotate when focusing.
The buid quality is very good even if it is 100% made of plastic. The zoom ring is fluid enough but there is no zoom creep when the lens is pointed down. There is a small button to lock the zoom but I have never used it.
The petal hood is heasy to mount and can be stored reverted on the zoom.
The autofocus is very fast and accurate even in low light conditions thanks to the wide aperture. I have checked that my copy does not have back or front focus problem (successfully) on a focus chart.
Because the purpose was to sell the 3 primes AF lenses to buy this Tamron, I compared the image quality (to check before to sell these prime lenses)
@24mm with the Sigma 24 wide II f/2.8. The Tamron was slightly better or equal to the Sigma 24 f/2.8 (so the Sigma had to go)
@50mm with the Minolta AF 50 f/1.7. This Minolta lens was slightly sharper @f2.8 but it was only visible in high contrast scene due to a better chromatic aberation management (PF). But the Tamron 17-50 can be considered as a sharp zoom even wide opened. In all other condition, I was not able to see a difference. (so the 2 AF 50 had to go)
In my opinion, it is a perfect zoom for landscape, thanks to it's sharpness and a perfect indoor zoom tkanks to it's constant f/2.8 wide aperture and fast autofocus. it can also be good for portraiture at 50m f/2.8 but I prefer other lenses for that (e.g. the Helios 44-2)
So as a conclusion, I sold my Sigma 24 Wide I f/2.8 and my 2 copies of Minolta 50 f/1.7 that were useless for me and the kit zoom. It was exactly for the price I paid for the Tamron. And the Tamron is new with a warranty. This is why I do not consider this zoom as out of scope there (I have not found a lowcost single zoom alternative)