Pentax 50-135 - why is it always

a tradeoff with everything in photography?  Is that a law?

I just received a 50-135 from Amazon...a "like new" warehouse deal (used) for around $850.

I am blown away by the optics, small size and build with weather seals.

But...you know what I'm going to say next...just too good to be true.

The AF is apparently dying on it.  Really skittish and sometimes just does nothing.

I've read about SDM failures being discussed here.  Dang...I just can't take the risk and need to send it back.  But the IQ really wows me.

I still can't get over the super high ISO capability of the K30...maybe I'll just get a 55-300 which is just a stop slower out to 135...anyone else made that call? Is the AF more reliable on that one?