Monster clean tone, no-nonsense controls (volume+tone), onboard tremolo. Great for Jazz and retro-rock styles (e.g. surf). Good base for a pedalboard. A one-trick pony, but does it well.
not a jet at all, actually a brownface fender deluxe with one channel... handwired by the hardworking people of wherever, this amp is a gem. Transformer set could be better and the stock no-name alnico speaker will not be to everyone's taste, but the amp is no jock. Only really serious flaw is a mistake on the input wiring. There's an easy fix that can be found with a simple google search. If you see a J20 jet do not hesitate....
I'm somewhat grateful that Ampeg's guitar amps of the last couple decades haven't been more successful, or these J-20s would be a lot more expensive than they are. What they lack in splash they make up for in substance and sound. The tone is amazing. And they have more headroom than you might expect from a 20-watt amp. When the volume knob reaches noon, the tone knob starts to behave almost like a gain knob. With the tone knob on the low side, the amp remains relatively clean. Turning it up (ostensibly increasing high-end, but not in any significant way to my ears) adds dirt. And the tremolo! The tremolo is pure butter.
I run a lot of pedals through this amp (technically, I should say "these amps" because I have two and I run them in stereo), and they sound great. But taking pedals is not necessarily what this amp is all about.