As a dealer I return many products to the manufacturers as a service to my customers. In two years as a Tamron dealer I have had absolutely no requests to return a lens Tamron for any reason at all, and never had a customer come to me with any dissatisfaction at all, even with their cheaper models. The one complaint I have had about Tamron is the "plasticy" feel of the barrel on their consumer level products and I agree with that comment completely.
I return one lens to Nikon about once every month or two (last one was for tight zoom on a DX 18-200mm, turned out to be dirt in the extending zoom mechanism).
I return 2 to 4 Canon lenses every month, almost always for the same problem, AF motor failure. Most of these lenses are over one year old and Canon will not cover the repair cost anyway due to their "shortest in the industry" lens warranty. All other manufacturers warranty their lenses for 5 or 6 years (Tamrons being the longest) while Canon only gives one year.
Internet "reviews" are almost never worth the paper they are written on......oh, thats right, no paper involved either. Take these review with a grain of salt. There is not a single product on the market that you cannot find horrific review on, if you want to search long enough. Each of these bad reviews is always offset with MANY more positive reviews every time.
And no one is EVER "better off" with a grey market lens, simply because Nikon will NEVER work on that lens (or grey market bodies) even if you offer to pay them double their labor rate, it just will not happen.