Hi all,
I'm a big lurker on these forums but then I saw this thread and thought I would make a contribution!
I have that Pentax 17mm-70mm f/4 and have used and tested it extensively. It's a great lens, it works well for landscape, portraiture and as a walk-about lens. I would be reluctant to use it for portraiture in low-light conditions, for that I would always use my 50mm f1.4. However I felt that I would contribute on some of the criticisms of the lens to clear up a few things.
Out-of-the-box, the autofocus doesn't do a great job. I first noticed it when taking shots at 70mm f/4 was everything seemed "soft" (consistent with the reviews of the lens). After a lot of testing at f/4, it was apparent that the AF wasn't getting the focusing correct. In order to get a good sharp picture at f/4, I had to go into the custom menu on my K20D and set it to around +5. I took many photos of the front of a book (my missus thought I was mad!) at each autofocus adjustment step and settled on +5. It's a superb lens, the SDM was great for a wedding I photographed recently.
Here are a few of my shots with this lens (the exif info may not be correct, I noticed it recorded the wrong focal length, need to check that out).
http://img154.imageshack.us/img154/7625/igp3706.jpg http://img503.imageshack.us/img503/817/igp3708.jpg http://img503.imageshack.us/img503/6859/igp3734.jpg
I don't care what a lens review tells me; I'm very happy with the sharpness, colour and contrast this lens delivers (besides, Photozone gave this lens a big thumbs up). Just make sure you spend time calibrating your autofocus for f/4 and you won't be disappointed.
If anyone wants any particular test shots at particular settings, let me know!