In brief: "Overall, the amp offers an array of neat features in an inflexible, cumbersome package most useful for guitarists who want specifically want an amp they can practice and gig with and who are prepared and willing to go the extra mile with the desktop software to get the most out of the amp. I would not recommend this amp for people who want decent tones from the source without the hassle."
The Boss Katana is probably the most hyped guitar amp on the market right now and I definitely bought into it once I had tried the amp at a local guitar store prior to ordering mine from Thomann. Especially the clean sounds I could get from the amp while playing a Fender strat blew me away and made me buy into the concept. Now, after having owned the amp for several months I've overcome the feeling of awe.
The vast array of features (and the quality of their execution) is perhaps the amp's number one selling point, which is fair enough. The amount of high quality stomp box effects included within the software of the Katana is indeed pretty impressive. However, the selection of amp profiles has turned out to be rather disappointing. While I've had the most joy playing through the "Clean" amp profile, the other profiles sound pretty bland, unimpressive and fizzy. The "Crunch" profile lacks crisp top-end or any kind of bite you would expect from a decent British style amp, whereas I struggle to distinguish the "Lead" and "Brown" profiles from one another. Both sound thin and particularly fizzy by themselves.
This brings us to the effects side of things. The variety of different effects available is plentyful and there is mostly everything you would expect with a few fun additions such as the synth effect or the human voice synthesizer. My biggest gripe with the effects, however, relates to the fact that any and all meaningful tinkering has to be done via the desktop app with the amp connected via USB.
This seems fairly standard practice but I find myself longing for an amp I can use without having to turn my PC on whenever I feel unhappy with the sounds -- something which seems to be a constant bother for me. For the few months I've owned the Katana, I've had it at home but were I to take it to a rehersal space I would have to bring a laptop with me in order to make any adjustments to the tone etc OR have to bring back the amp home every time. With the 50 watt model only supporting four programmable soundbanks the tonal variety independent of desktop access is very limited. So, in case you want to dial more than four different soundbanks on the go you'd rather buy the 100 watt specimen.
The free, desktop software provided by Boss for the Mk2 is alright. The control panel where you can edit the soundbanks etc does an alright job and offers yet again a variety of options to alter the sounds available to you. You can also import (and export) soundbanks you've created with others. However, this feels like one of the most undercooked features on the Katana ecosystem. The application itself doesn't feature a library for custom soundbanks which feels like a massive missed opportunity. Instead, there are tonesets you can download online by doing a little bit of googling. However, most of the ones I've downloaded and tried out (and trust me, I've gone through quite a few of them) have varied from very poor to underwhelming. Also, the library interface for organising your imported tonesets is absolutely jarring, organising these tonesets consisting usually of 1-4 different preset banks into vertical columns, resulting in a very counterintuitive and off-putting layout. If the content was bad then the UI to browse this content is somehow way worse.
The variety of effects such as boost pedals, overdrives and distortions do not ammend the fact that all the distorted sounds you dial in sound thin, fizzy, bland and generic. This applies to the channels apart from "Clean" (and "Acoustic" which I haven't really used). My favourite tones are all derived from starting with the clean channel and adding some gain until the tone starts breaking up and then adding your preferred stompbox of choice. This method offers some decent options for glassy-ish lead, fuzz or even Marshall-esque tones.
The physical FX adjustments on the amp are by far not my favourite feature. The double layer selector knobs are annoying to use as they are very small and I constantly keep forgetting what effects are stored in which one, as there is no kind of screen indicating anything. This makes the effects seem rather gimmicky and a poor replacement for actual pedal effects.
In general, unless I had very limited tonal requirements I would not gig with this amp. Sure, the wattage would suffice for pretty much any conceivable kind of gig, but personally I prefer not having to bring a PC on-stage. Overall, the amp offers an array of neat features in an inflexible, cumbersome package most useful for guitarists who want specifically want an amp they can practice and gig with and who are prepared and willing to go the extra mile with the desktop software to get the most out of the amp. I would not recommend this amp for people who want decent tones from the source without the hassle.