I know Mumsnetters have conniptions at the thought of any kind of fake smell, but these things are in the shops so some of us are buying them.
I'm not sure I've ever deliberately bought one, but I definitely seem to own one once in a while. It's a pretty bottle of smelly oil with sticks. They always smell nice for a bit and then stop smelling and then sit there getting dusty while the oil gradually evaporates until I throw them away. But today I learned you're supposed to flip the sticks upside down every few weeks. It actually works!!
So, if you've got one and it's stopped smelling - try flipping the sticks.