I use my Sigma HSM OS 17-70 very often on my K5. The in-lens OS appears slightly better than the camera SR, but so little that I usually just use the SR in the body.
Some tests I saw here a while back called them pretty much equal....I wouldn't argue with that.
Where the SR shines is on lenses that have no OS......the SR makes a huge difference. You will get shots you never thought possible. Like this one hand held at 500mm 1/20.......Without SR even my steady hands could never get this one, shot through double pane glass at about 30 feet distance with the K10D. I think the K5 is even better!
Best Regards & Enjoy your K5, it is a marvel of a camera!