I have a DA70mm, and love it.
I used to think that anything longer than about 135mm (35mm equivalent) was too long, but lately I've been rethinking this lens philosophy.
Here's an article by Ken Rockwell to read, and to discuss.
Portrait Lenses
I know some people like Ken's opinions, and some don't. I only posted this article, because I have personally seen fashion photographers here in New York City use very long, fast lenses to get tight head shots and make the background disappear. He states this in his article.
I plan to test out my Pentax M100mm f4.0 Macro and newly acquired Takumar 150mm f4.0 for portraits, since the weather is getting nicer now.
They will yield fields of view of 150mm and 225mm focal lengths respectively.
We shall see.
Mike