DS has never had one and we only used Panadol for his first two teeth (that came through at once at 4 months). I looked into it at one stage, but since teeth haven't really ever bothered him, there's been no need. I think, if you like the look of them, buy them for that reason, and if they work for teething, it's a bonus! As a pp said, you'll never really know if they teethe well if it's nature or the necklace, but, who cares?!
BTW, a lot of people have said to me, "I'm glad he doesn't wear one of those awful teething necklaces like a lot of babies do these days" - lol! Obviously they have no manners, but just wanted to say that not everyone thinks they're cute!