The SS-1 case does have a couple of limitations. It has only two buttons, so the only camera controls you can operate while using the case are the on/off button and the shutter. You'll need to set your camera on full automatic and select any underwater scene mode it may have before putting it in the case. Custom-made underwater cases usually have multiple buttons to allow almost full camera operation while underwater, but this would be impractical given the universal nature of the SS-1, and also immensely fiddly to set up.
Another limitation is that for the majority of cameras it will not be possible to use a built-in flash, so this will have to be switched off. Again, many custom-made dive cases include prism systems and diffusers to allow underwater flash photography using the camera's built-in flash, but with the SS-1 using the flash on the Casio EX-S12 just caused internal reflections that ruined the picture. This isn't actually as much of a problem as it might sound, because using a camera-mounted flashgun underwater usually just results in a lot of back-scatter from suspended particles in the water. It's a much better idea to use a high ISO setting or a longer shutter speed, especially since camera shake is less of a problem when vibrations are damped by the surrounding water.
In actual use the SS-1 works extremely well. I'm not trained in SCUBA diving, but I took it snorkelling in the sea off the Devon coast and found it very easy to use. With the tiny and very light Casio EX-S12 inside the case was buoyant enough to float, but I suspect it would be possible to achieve neutral buoyancy by adding a couple of shot bags. As for picture results, obviously that will depend on the camera that you put inside it, and also the clarity of the water. The sea around Britain is notoriously cloudy, so I was only able to take a few fairly close-range shots, but the S12 has an underwater scene mode which produced good results.
The Seashell SS-1 (and its stablemate the SS-2) provides a simple but brilliant solution for underwater photography using your everyday compact camera, and costs considerably less than a custom-made dive case. It is extremely well made, cleverly designed and reasonably easy to fit to almost any camera. It has some limitations, but overall it works like a charm.