I've used them on my Martin O and found that they were wonderful in terms of tone and volume. Intonation proved to be a difficulty, which was always something I understood would be a problem with the C string, and it was. I eventually replaced them with Worth clear mediums when they lost their zing, which took about nine months. Not bad at all -
however, I've had a set of Aquila natural guts on my Maybell model 20 banjo uke and they've lasted about a year with little appreciable loss in tone. Also, no intonation problems of any kind with this uke, which has extremely low action. Frankly, I don't think anything sounds quite as good as a banjo uke with gut strings. Just amazing.