I have had the 50-135mm lens for well over a year. Like many people, I had read the reports of SDM failure but still went ahead and bought the lens anyway..
. I did get a 3 year Mack Extended Warranty though; as it was pretty cheap. Not had a problem so far but that in itself does not mean much either way.
Why did I buy this lens despite all the reports?
The main reason is that I had rented this lens for 2 weeks and really liked it as I got pretty nice shots of my kids in dances, sports etc. It was much lighter and smaller than some of the 70-200 lenses that were alternatives. I never thought of the sigma 50-150 because I never knew about it and still dont know if it is good or not. My final reason was that I just decided to 'gamble'. I thought to myself, "The kids are growing up quickly, if I can get 3-4 years out of this lens for $750, I will not complain! By that time, the kids are not going to do much of these events anymore and I may not need this lens if it conks out". This is not a great argument but that is how I rationalized it to my wife: 'We replaced our $1000 computer in 3 years and gave it to charity for nothing, I am going to get some permanent memories for much less"
Well, what can I say, she bought it! I am not saying anyone should buy this lens with this argument. But...so far so good with my copy enjoying this lens BUT keeping my fingers crossed!
Should pentax users have to put up with all this stress? No. Should pentax offer better warranties for sdm lenses. Yes. But that is neither here nor there. At the end of the day, from all the reports out there, I think anyone who buys the sdm lenses maybe taking a risk. How big that risk is unfortunately cannot be quantified becuase there is not the kind of data available that one can use to make such an educated guess.
Anyway that is my 2 cents.