So, you've had grief with two pre-Global Vision lenses and have seen complaints about other lenses you haven't tried...(complaints that haven't shown up in any of the reviews I've read.) Based on this you feel qualified to contradict the premise of this thread?
Here's a review of the most recent 18-35 f/1.8 lens:http://www.dpreview.com/lensreviews/sigma-18-35-1-8/5
"We shot with a range of Canon SLRs, from the entry-level EOS 100D to the top-of-the-line EOS 7D, and all had problems focusing absolutely correctly all the time
. This was usually only obvious when shooting at apertures larger than F2.8...
We looked at whether the focus problems we saw from the 18-35mm could be mitigated by using autofocus microadjustments. With a Sigma USB Dock to hand, we set about determining and programming in a full set of autofocus microadjust parameters for all of Sigma's specified focal lengths and focus distances (18, 24, 28 and 35mm; infinity, 0.5m, 0.35m and 0.28m). This took several hours
to set up, even with specialised focusing targets to hand...
This procedure certainly improved overall focus accuracy when shooting at the distances used for microadjustment. However these are fixed by the software, and there's no option to specifically correct any distance between infinity and 0.5m. Unfortunately though, the vast majority of subjects end up somewhere in between, and we found that the lens still had some problems with focus accuracy even when fully programmed as above
My experience with the updated Sigma 30 f/1.4 Art lens mirrors DPReview's article above.