I have used it on K100D, K10D and finally on K20D.
It never failed my expectations.
I love that lens and I believe you will also like it.
Its maximum aperture is 2.8, hence a brighter viewfinder...
Its range extends to 70mm, hence it covers most of the portrait range...
It allows closer focus. so you can shoot quasi-macro photos.
It is sturdy enough.
Vignetting is noticeable (though, not disturbing) only in its <20mm setting.
It is not that BIG.
And the below are the cons!:
It is not the best lens out there!
It is not the cheapest lens of its kind.
It is not an f/2.8 lens throughout its range.
Its tele end does not reach 100mm and 135mm and ...
Seriously though, probably no zoom can deliver results (under certain conditions) as a good prime.
Sigma 17-70mm is certainly a good general purpose zoom lens.
Happy shooting in 2010