Based on seat of my pants calculations it seems like these lenses have a problem in about one out of three cases. Does that seem fair?
not really you are taking an extremely small sample group from an internet forum there are probably many many more buyers that dont come here who knows what their experiences are
True. That's the problem with a small, non-representative sample size like CR forums. For all we know, it could be that everyone in the world who has gotten a good Sigma lens is posting here, and 99.99% of lenses out there have problems.
haha entirely possible
I have 3 sigma lenses actually had 4 including the previous sigma 50 which i gave away after i got the art but it was pretty flakey on my bodies however it seemed fine on my wifes 5Dmk3 though...
the 85 which i love was my first and out of the box it was horribly back focusing the the degree of the 13 adjustment in AFMA, i sent it in to sigma and it was back in under week and was spot on i dont have that lens set to any AFMA value on any camera and its been used on 2 5Dmk2 bodies 2 5Dmk3 a 1Dmk3 a 600D a 550D and 2 different EOS-M bodies all are fine. it does however miss focus occasionally but not very often.
the 35 was perfect out of the box is amazing tracks so fast is incredibly accurate and just mindblowingly good it just never misses. EVER
the 50 ART was off and i knew it as soon as i picked it up in the store nut hey it was the first and only copy in china so i didnt have alot of choice, i bought the dock and calibrated it and it seems fine but it still misses sometimes but is FAR and away more reliable than the 50 1.4 canon or sigma old version or the 1.8 canon (I never bought the 1.2 but tried it many times and never could feel it was worth the price)
after calibration the 50 ART is pretty great and I've been using it alot its an amazing piece of glass especially considering the price.
So while i'm a fan of sigma primes I full agree that they definitely have a high frequency of issues out of the box but with some perseverance these can be fixed.