I have never tried one but think they are more novelty value than anything else and expensive. We chose a £1 potty from asda and dd was happy to use this.it also meant we could take it out with us which you won't be able to do with a musical potty. You could try stickers after each successful attempt on the potty as an incentive but I personally wouldn't buy one :-)
Just don't. We have one and it is such a hassle to clean and not once have we heard it make music (maybe we never put batteries in it? I don't know, I couldn't be bothered to read the instructions for a potty).
Anyway, all of my children were potty trained before the age of two without the need for a musical potty.