Thanks for the replies, team
I guess the final arbiter is whether or not the final image is any BETTER with in-camera, in-lens, neither, or both. I seem to get pretty good results handheld with the in-camera SR on my K10D and K20D, but I haven't tried any stabilized lenses, none so far being available here in Australia that I'm aware of.
EDIT: I see that the Sigma 18-250 marketed here now has OS, but I don't have access to one to try it out.
I'm assuming that as we already have in-camera stabilization, then the benefits of in-lens stabilization would be marginal at best, if we have to turn off the in-camera SR. The difference would be between the two stabilization systems, NOT between in-lens or NO stabilization as on CaNikon and others.