The Tamron 24-135mm was a lens I purchased a while back in preparation for going full frame, but hadn't used it a lot until my recent trip to Australia. Though it has no stabilizer or in built motor, I had read very good reviews on lens performance, and they can be purchased quite cheaply. The 24-135mm zoom range is also very useful for an all round type lens, which makes it very appealing.
To look at and feel, the lens is quite solid and heavy, like real metal and glass have been used. The build quality is also very good and the zoom mechanism doesn't ever wobble. However the weight does mean it is a bit of a pain to carry around when street shooting.
At maximum aperture the performance is good sharpness wise. It is relatively sharp in the centre, with the corners a little behind. However there is very little light fall off, and performance is pretty consistent right through out the zoom range.
At f8 though, this lens really ratchets up the performance. At most zoom length it can out resolve the 24mp sensor of the Nikon D600, which is no mean feat for a 5.5x zoom lens. In fact the shots it can turn in are really quite impressive.
All in all, this is a very good lens. If you can live without VR or AF-S, its a true bargain.