This is a weird one - in many ways ! First off - I put this on my pedalboard in place of a compressor / sustain (only ever used in sustain mode). Nothing happened - it didn't even light up any LED's no matter how much I tried pressing the button ! Have I got a dud ? - THINK ! - read the spec. - this requires 235mA of current at 9V. I checked the power leads back from my power supply (now THAT isn't lacking in current !) and found I was only giving it 100mA - far too little. - Just swapped over to an output giving 250mA - I could have used one of the 400mA outputs just as easily. Now everything was working fine. Don't return your pedal - Check your power is sufficient first !
This didn't really sound much like feedback to me - and it ONLY appears to work on single notes ! - If you play chords it latches on to the lowest bass note and creates harmonics from that. (looks like the same processing algorithm used in the Vocal Harmony effect !), Fine for soloing, but can be rather strange if you hit chords. The online demos appear to emphasise the non-latching mode a lot and it works well in this mode.
The low octave is particularly unususal and sounds very smooth, - almost a pure sine wave. This is the cleanest sub-octave I have ever encountered! - but only monophonic !
On a bass guitar this effect is in a class of its own and you need speakers that can handle it.
All the sounds are quite pleasant and musical - just not really "Proper" traditional sustain or feedback. Considering the price, I can't really complain, and this is an unusual, but musical effect. -
One or two part harmony, but adapted for guitar. It really ISN'T a sustain sound - its a SYNTH !