http://www.the-digital-picture.com/R...ns-Review.aspx
IS (Image Stabilization) makes getting sharp images while handholding the Canon EF 300mm f/2.8 L IS USM Lens significantly easier for subjects that are not moving rapidly - or are able to be shot in the available IS Mode 2 (Panning Mode). IS allows handholding this 300 at shutter speeds of 1/125 or lower (about 2 stops of benefit). I am a huge fan of Canon's IS lenses - they definitely let me get shots I would otherwise miss.
The IS version on this lens is tripod-sensing. The Canon EF 300mm f/2.8 L IS USM Lens knows that a tripod is being used when vibrations go below a certain level. Keep IS turned on when mounting this lens on a tripod to take advantage of the secondary IS mode - reducing mirror slap, shutter and tripod vibrations.