A few years ago I swore off of third party lenses and since then have purchased nothing but Nikkor. I have to admit back then I did so more out of a desire to improve my overall kit and with the assumption, not based on fact, that OEM lenses would help. Recently the need to replace my APS-C format 16-85 Nikkor lens put me in the market for a similar focal range. Since I was there anyway I figured I might as well look for something that would be an optical upgrade. Among the available factual data that I could find available (i.e. test scores, not just subjective reviews), several third party lenses kept popping up purely for technical performance without consideration of cost.
To make a long story short, I ended up purchasing a Sigma 17-50mm 2.8. I haven't done any serious work with it yet but have to say that for overall build quality, AF functionality, etc., I 'm pretty impressed. Sigma has come a long way since my last one and for any Nikkor snobs like me, I highly recommend giving some of the newer Sigma models a look.
The Sigma lens I was really impressed with was the APS-C format 18-35 1.8. But I was looking for the proverbial travel/walkaround lens and the 18-35 is a little to restrictive, is HUGE and has no VR/OS for shooting handheld.
When I get some useful results with the new lens I'll report back and post a few. May be a while though as I have a lot of both business and personal travel coming up in the next few weeks.