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.

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I'm tempted to add that there are a number of m 4/3 lenses that can be pixel peeped as far as you like. I do wonder how long this will last though. They need to keep it up really as the sensor is smaller but if people are just going to post uncropped images 1000px or even smaller on the web why bother? Even Nikon are providing extra resolution on the even smaller V range cameras.

