Considering this amp is about the size of a toaster, it is pretty damn impressive. Far more capable than its size would suggest.
Build quality is superb - there's no creaking, flexing, or "cheap" feel to it whatsoever. It looks and feels every bit as premium as it's larger siblings. Fit & finish is immaculate.
In terms of volume, it's basically black magic. It is remarkably loud yet clear considering how small it is. I tend to play it with the volume set to 9 o'clock (so around ~20% of max volume), and that is more than enough for it to adequately flood my home office. I will say that it can "fart" a little bit at the higher volumes (dependant on your gain & EQ), but at those volume levels it is far too loud for home practice use anyway. Probably not suitable for gigging though... besides busking, which it seems reasonably ideal for.
The tones on offer are the real surprise here though - and exceeded all expectations in my case. It sounds great. It might've been nice to have a couple more preamp types, but you do have a total of 6 to choose from (3 distinct preamp settings, each with a variation), with an ANALOG gain circuit, serviceable (but not great) EQ, and of course the optional effect and reverb settings. Obviously it can't compete with larger and more feature-packed amps, such as the larger Katanas, but it is pretty remarkable in its own right. Positioning is important though - it sounds MUCH better around waist height than sat on the floor. I keep mine on my desk, which suits it nicely. And it hardly takes up any space!
On top of all that, it has a built-in tuner (which is accurate, but sadly not chromatic), bluetooth, stereo 3.5mm out for headphones and/or recording, and it lasts a loooong time on a single battery charge - easily up to 10 hours playtime.
Overall, I am thoroughly impressed with the Mini X. It is not perfect (maybe a future third iteration of the Mini will give it the finishing touches, but that could be many years away if it ever comes to fruition), but it is about as close you can get to a "perfect" compact home practice amp. It also makes for a nice desktop companion to your PC's audio interface (if you have one).
Highly recommended. Just don't expect to gig with it!