An internet search reveals various specialist reviews that'll either rate this lens as ahead of the equivalent camera manufacturers' lenses or slightly behind them in terms of quality. That's an ambivalence that points to pretty good glass and at a price which is significantly less than the camera brand-name - the product almost sells itself.
I've had this lens about 8 months now and it's probably on my Nikon D80 about 60% of the time. It's a great all-round lens and in my view its quality is easily as good as the first-generation equivalent Nikkor (I don't know about the latest Nikkor version). It's well built (maybe even a little heavy) and exudes quality. The internal motor is virtually silent and the optical stabilisation only makes itself known by the little clicks you hear as the gyroscopes kick in - and make no mistake about it, the stabilisation works! I was a little sceptical of its efficacy when I got the camera as it was so quiet, I wondered if I had a faulty model, however after a half-hour testing it at various focal lengths, apertures and shutter speeds and comparing the results with the OS switched on and off, it became readily apparent that it does significantly help at those longer focal lengths and/or longer exposures.
For all that, you have to bear in mind the compromises involved in building a lens with this range. It's like the proverbial "jack-of-all-trades-master-of-none". It does most things pretty well without being outstanding at anything. It can be a little soft wide open and it's not a fast lens (although the OS helps), and if you like soft, creamy bokeh this is not going to deliver.
Having said all that.........would I buy another? Definitely! It's a great all-rounder and has come into it's own when I've been on overseas trips where it's impractical or ill-advised to take anything more than the bare minimum of kit. It's a great walk-about lens too. Whack it on the camera and you've got a lens that will deal with the vast majority of photographic challenges.