They absolutely work!!
My Boyo got his at about 12 weeks...and has worn it ever since (3 in April).

I took it off one day to give it a clean, and misplaced it, he was without it for a week, the change in his behaviour was incredible.

Then I found it, put it back ok and even the girls at the creche he goes to asked what had been wrong the week before (necklace free), and were so glad to have the old Boyo back (necklace on!)