They are AWESOME! Quite simply they stop the shaking and movement.

With any binocular or telescope -- as you increase magnification the effects of waves and boat/body movement are also magnified. That's why good marine binos are usually 7x and not higher power. If you increase magnification more than that you have diminishing returns in stability so the net effect is not an improvement in perceived real-world detail (e.g. ship names or buoy markings). My 14x40 Image Stabilized (IS) binos are much more stable than 7x50 standard binos. The Fuji optics are also excellent.

I haven't tried any other brand, but the Fuji Techno Stabi are effective and responsive in any attitude from looking downward to horizontal to looking vertical (great for star gazing). When in use, I "sight" the subject I want to view with the IS function turned off, then "lock-on" by pushing the IS button on top of the binocular. Once locked on, there is a bit of a lag when you pan the horizon or vertical axis because the IS function is trying to keep a specific fixed spot in view, but you get used to it and slow panning isn't an issue. You're supposed to push the button again to turn off IS and lock the internal mechanical mechanism before you put them down or put them away. They're completely waterproof and submersible IPX-7.