I find the in-body stabilization at least as good as Nikon's VR II. I most use my E-M5 with Nikon and leica manual focus lenses. You just enter the lens focal length in the IS menu and you're all set. I don't have too many photos with really slow shutter speeds but here are a couple I took out of my kitchen window. Nothing special.
This one was shot with the Nikon AIS 200mm f4 at 1/10 sec. If you could see it at 100% you'd see a little movement so It's about 1 stop too slow.
This one was taken with the AF 85mm f1.4 at f2.8 and 1/15 sec.