The plugs are quite fragile: they are soldered directly to the board and not attached to the case, so a mild knock on the headphone or Aux plugs can detach the plugs and require tricky soldering to fix (I had to do it twice, then it died for good).
The amplifier is very noisy. The hiss would make my ears ache after each use.
If you turn down the tone control, the hiss goes away, but so do all your highs and mids.
Selecting a rhythm is easy, and the built-in metronome is handy, but adjusting the tempo requires holding down a button AND turning a knob at the same time, one-handed, which is very difficult. If you slip, you either change rhythm, or change its volume instead.
The three gain levels didn't do much, and felt too quiet except at max.
However, the battery life is quite long, at least 10-15 hours on a pair of AAA batteries.