IS is super-helpful when you shoot in the wind, like say at windsurfing events, or at the seashsore where gusting winds are the norm on many days. It is also useful when shooting from moving boats, or boats on ocean or other wavy bodies of water, like say The Strait of Juan de Fuca or the open ocean, where there is a LOT of irregular and basically impossible-to-predict movement due to wave or wind-and-wave motion. IS is useful when you do a lot of slow-speed panning, since it can cancel out the up-and-down movement. It's also useful when you are shooting when you might be tired, or somewhat out of breath. It's also useful when you want to shoot hand-held stopped down to smallish f/stops, to get good depth of field, without needing a tripod.
In other words, it's hugely useful. Buy it if you can.
IS or the Boat?