Here's some
reviews from Fredmiranda.com on the 135-400.
As for the 170-500, it's kind of soft at the long end. I have had this lens since last year when I bought it used from KEH for about $350. If your subject is going to fill the frame, it works fine. But if you do a 100% crop, the softness becomes noticeable. That said, I have gotten acceptably sharp shots with this and my K10D as long as I was using a monopod or my tripod. If you are considering this lens, I would recommend saving up a bit more and go for the Sigma 50-500. If I were considering buying both lenses band new, I would go for the 50-500 for sure. The 170-500 was a great buy at $350, but not at the $669 at some places.