Does anybody have any experience of using one of these on a Pentax body? I was considering getting one for my K5 as I like to do wildlife (mainly bird) photography in the local area. I don't think I'd use it a huge amount, but I really fancy being able to get much closer than my current 70-300 allows. Any views on this lens? I can get it for a fairly good price just now so I am tempted (£600).