I've had mine for a couple of weeks now.
It's a great bit of kit, for anyone who's had any experience of the Katana guitar or bass amps this is more of the same in a tiny plug in and play package.
I will say the existing bass presets aren't that useful, 3 or 4 of them have 'slap' in their description and having determined that they weren't particularly good for slap or any other context those were the first slots I replaced with my my own patches.
It probably took me under 5 minutes to create 2 new bass patches, one clean with a bit of compression for a 'modern tone' and another with a bit more grit.
As I mentioned in another post I have the guitar amp, but I reckon the Katana Go app is intuitive enough that even anyone completely new to the brand will be able to dial in something they're happy with within a few minutes of downloading the phone app
As far as I can see you do need the phone app to set up new patches, it gives you easy access to the different amps and a fairly extensive array of effects where you can adjust all the parameters you'd expect on any amp or physical effects pedal.
The Go has 30 slots for your patches, which is less than than the average multi fx unit, but this isn't trying to replace the Helix in your main rig, I reckon 30 is more thn enough for practice, especially as it's so quick and easy to set new patches up or tinker with the ones you've got.
Once you've created the patches you want to use it's easy to cycle through them and the neat little display window will tell you which patch you're on.
There's also a nifty tuner function which, again is displayed on the device itself and does not require the phone app.
I've not had much experience with headphone practice amps, but I will say that the GO probably sounds as good as any full sized amp I've ever plugged headphones into, with the caveat that the headphones I use for practice are good quality over ear jobs (Behringer 770 Pro) I'm not sure using cheap earbuds , for example, would give the same experience.
All in all one of the better purchases I've made in recent years. It's just the ideal tool for those moments when get the urge to pick up an instrument and just start playing.