I bought this on a lark and it works as advertised, but I have only one actual use for it: 
- When warming up at a jazz gig or rehearsal. Then, I will 99% of the time be carrying my harmon mute. Better than carrying a full-size practice mute. 
 
It is not useful when: 
- I have a gig or rehearsal with symphony or wind band. They don't call for the harmon. 
- I am practising at home. The harmon + mini is too heavy, so it is uncomfortable for long sessions. 
- Travelling, because it's lighter to just pack a regular practice mute. 
- Buzzing - because I don't believe in buzzing as a part of practice. Just a few seconds to get the lips started before warmup.