So far so good. They've lasted at least a fishing season. They were used to splice in new nav lights on a friends boat, and I used them on mine when running new wires to my washdown pump and bilge pump. The connections for the bilge pump were down in the bilge area, so I wanted the sealing feature so moisture wouldn't get into the wire.
Interesting. My preference for sealing is the glue-filled heat shrink. It is a real bugger to get off if you need to redo it, but it keeps things real nice.