Literally, the best thing ever. I use my everyday.
The actual burner costs about £10 – £13, depending on where bought from, you can get different designs and patterned things but apparently this one is one of the better made ones. Check out yankee candles website to have a browse. Then the little insert bits cost £1.25 which I think is so reasonable. You can keep burning it until it evaporates, and they take ages to completely burn down, and I mean ages. If you get bored of one smell then you just wait till it cools down, take it out and pop another one in.