I've tried them both plastic as well as metal. (If I had to use them, I'd lean towards the plastic ones.) They take some getting used to, and weren't my personal cup of tea. In the end I either play bare fingered, or thumbpick and bare fingers (with or without nails - doesn't matter) or using fingerpicks on thumb middle, index, and occasionally ring.

But the thing I find most paradoxical about them, is that you have to have some existing nail already happening in order to be able to "anchor" them - and if you already have nails, then you shouldn't need them, (unless of course your nails are so fragile that they break or crack under the strain of pickin'.)