They're wonderful. They do darken the water; many aquarists like the aesthetics of a crystal clear tank, but bettas like the dark water. How dark they tint the water depends on how many/what size leaves you use. Catappa leaves help simulate the betta's natural environment, and they release tannins, which help prevent illnesses and heal injuries, into the water. I've never seen a betta ever try to nibble on them; I think their effects are just on the water itself.
Some use the leaves to brew a dark water extract "tea," some put them in the filter, and some just place the leaves in the tank. I don't mind the look of the leaves and I'm lazy, so I just drop them in the tank and leave them in until they start to get slimey, then I change them. My bettas like to swim around and underneath them, and I think they (along with frequent water changes of course!) are really helping prevent my tail biter from getting fin rot. I highly recommend them!