[bc_the_path]

As a former (happy) user of Sigma's versatile 17-70mm f/2.8-4.5 lens; I have just got the new offering with fixed maximum aperture today. I will report my experiences here soon (perhaps reviving this thread).
In the meantime, you can perhaps look at the two reviews about this new kid on the block (on different mounts):
1) Photozone:
Sigma AF 17-50mm f/2.8 HSM DC EX OS - Review / Test Report

2) Lenstip (a useful Polish site) :
Sigma 17-50 mm f/2.8 EX DC OS HSM review - Introduction - Lenstip.com

A comparative analysis with Tamron 17-50mm f/2.8 would be nice (as the Pentax lens with the similar range is pricier than both). I believe the reported user experiences with the Tamron was very good.

My first impression is positive and the lens seems to be focusing normally (no front- or backfocus). More will follow

[pop4]

Yay, another person with this lens. I posted a few of my initial thoughts about the lens here: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/114912-sigma-1...le-pentax.html
Hmm, I probably should update that

[bc_the_path]

- There is no front- or back focus.
- The lens is internally focusing. The length changes only with the focal length.
- AF is fast. Really, very fast (and, naturally, quiet). This is due in part to the very short focus travel; about 30 degrees, perhaps. So, from 30cm to infinity the barrel takes not more than a 1/10th of a full turn.
- MF is possible but unlikely to be precise, see above.
- Optical stabilization (turning the SR off) is very good and probably a bit better than the in-body system. However I have not done any systematic testing. I already got the impression that OS may be consuming a bit more battery power than the SR. Time will tell.
- The barrel does move during AF but this is hardly a problem since the travel is usually a few millimeters only.
- Resolution and contrast are very good (again, without formal testing yet).