If you need something to make your electric guitar or bass _audible_, without soaking up lots of room or all your checked-luggage allowance, get this in combination with an Amplug.
Gotchas:
Make sure you feed it a battery with _at least_ 9v. With the stock battery, my bass produced nasty distortion. When I switched that to a 9.6v rechargeable, it cleaned right up and sounded, well, as good as you can hope for from a 1-inch cone.
Remember to switch it off when you're done. Easy to forget, and the other reason I'm glad I use rechargeables.
Most useful for: fretless bass with flatwound strings, trying to keep up with acoustic instruments and voice.
The built-in connector cable is a little fiddly and will probably be the first thing to break. That was the only reason I saw to knock off a star.
Features? Well, it takes the output of an Amplug and makes it louder. That's what you're paying for, and that's what you get. You also get the sight of a teeny-tiny stack, if you squint a bit and possibly have a couple of drinks first.