The optical image stabilization system used on the G11 works pretty well for most everyday photography - though of course it can only compensate for shake, it won't make your subjects sit still at low shutter speeds. There are three modes: Continuous (IS on all the time), 'Shoot only' (IS is activated at the moment the exposure is made) and Panning (for horizontally panned shots).
The first option makes framing easier - the IS system steadies the preview image, and we found it more consistently effective at extremes (where the shutter speed is more than 3 stops slower than would normally be necessary for the focal length you're shooting at) than the Shoot only mode. Shoot only - which doesn't steady the preview image - uses less battery and still offers a considerable improvement over no IS.
The stabilization test
In this simplified version of our SLR IS test, ten hand-held shots were taken of a static scene with the stabilization off and on. The shutter speed was decreased and repeated (from 1/125 sec to 1/2 sec). The zoom was set to its maximum position (140mm equiv.), the test target was 3 m away from the camera. The test was repeated three times and an average taken.
The resulting images were then inspected and given a blur score - 'Sharp' (no visible blurring at 100%), 'Mild Blur' (the kind of camera shake that is tolerable at small print sizes) and 'Heavy Blur' (virtually unusable due to camera shake) and 'Very Heavy Blur' (little discernible detail).
As the charts below show the IS system does give you about a three stop (almost four stops in continuous IS mode) advantage over shooting without IS, and even dropping down to four stops below the recommended minimum for a 140mm lens 90 percent of shots were sharp (again continuous IS mode).
Hand-held, no stabilization (140mm equiv.)
As you can see from the chart below even at 1/60th second it was possible to get most of the shots taken to be completely sharp. (These results should not be compared to the G10 figures, partly because the different shooters may be more or less prone to shake but also because the G10's higher resolution shows shake more readily). Instead these figures should be taken as the benchmark over which the IS system needs to show an improvement.
Hand-held, stabilization continuous (140mm equiv.)
With stabilization on the results are much better - we got no blurred shots at all at or above 1/15th sec, and it's not until 1/4 sec (around four stops below what the rather approximate 1/equivalent focal length rule would advise), that large numbers of shots are blurry. More to the point, this performance is around four stops better than we were able to achieve with IS turned off.
Hand-held, stabilization single (140mm equiv.)
Switching IS onto 'Shoot Only' mode helps to save battery life by only activating the stabilization system when the shutter button is half-pressed. The results aren't quite as impressive and in 'Continuous' mode but still offer around around a two-stop improvement when compared to shooting with IS turned off. We'd tend to use this mode most of the time, switching into 'Continuous' when in a particularly challenging situation.