I wanted to visit Lumiang Cave after a friend recommended it to me. I think it's a good idea to do the tour in the company of a good guide as the relics have been left to abandon somewhat and a guide will be able to prevent them deteriorating too much (though I observe some of them encouraging tourists to actually touch the relics).
The coffins are piled up as if they're just pieces of stone or something, and that creates a bizarre impression. For me, however, the highlight of the tour was the experience of negotiating the tunnel that connects the two caves, i.e. connects the Lumiang one to the Sumaguing one. It's something I definitely recommend doing, as long as you're in good shape and not claustrophobic. Though you will need the services of a guide, these will be the most adventurous four hours of your trip to the Philippines! Expect to be plunged into darkness, to get wet, and to have to climb onto your guide's back at times.