Being that the Ray o Vac hybrids are still 1.2 volts just like regular NIMH and Nicads, you can expect them to show a lower voltage right off the bat. However, they lose much less of their charge in non-use and that is why they can put them on the shelf ready to use. I've been using them exclusively in my digital camera with excellent result so far. As to how long they will last in a detector, I can't say for sure, but I'm thinking they will hold up for several or more fairly sincere hunts and as long as you carry spares, you should never have a problem. I think most of us carry spares no matter what type of battery we use, anyway.
Dig often, dig deep,
Bill