I've had the Aspri reverb unit for several years and use it on a nylon string flamingo Alverez. I does so well I leave it on all the time. It gives a noticeable echo/reverb effect and I want to find another one because I believe the springs are starting to get weak after being left on the guitar all these years. I have five other guitars, Martin HD28, Gibson Heritage Custom, Gretsch Country Gent, Lone Star Dallas and a cheap old Silvertone archtop and a home made banjo and have played for 40 years (since 14). After using it all these years I'm suprised no one else uses them or seem to have heard of them. They were advertised to get you used to the echo effect in a concert hall.