Ok, I put this thing through its paces. I tried it with my Fender American Original 60s Jazzmaster and my Heatley Tradition (think Les Paul, but nicer).
The presets could be better. The stock presets seem to work best with humbuckers but qualitywise are all over the place, from ok to just bad. Lack of good categorization (despite seeming to have support for folders) does not help. I'd recommend just dialing your own using the Default preset.
But the good stuff: It sounds good!
I liked the EL34 amp much better than the 6L6 model. I thought the only difference between these amps was styling and power tubes but it seems the 6L6 is a lot higher gain and geared towards hard rock and metal whereas the EL34 is more blues, classic rock etc.
The EL34 seems more flexible and sounds pretty nice whether it's for clean, edge of breakup, crunch or distorted tones. It doesn't have a huge ton of gain on tap, but using the Drive effect pushes it nicely.
The 6L6 did not like my Jazzmaster at all. It just sounded bad with it, no matter how I dialed it. The Heatley fared much better, so maybe the 6L6 is better suited for humbuckers? In any case, this is in a modded Marshall vein and can sound pretty mean. The 3-band EQ seems pretty effective.
The effects sound good. They are not going to blow your socks off or anything, but there is also nothing wrong with them. For reasons unknown, Flanger is in the post-fx section when it could easily be in both pre and post, similarly Chorus could be in post too but is only in Pre-fx.
The cab sim is limited, with a bit cumbersome UI. No adjustable mic position beyond on vs off-axis but you can mix two different cabs together with a different mic on each and pan them.
Low/high cuts being in the EQ department after all the effects is not my preference. I think swapping EQ and Post-Fx placement would work better. Anyway, the EQ block is nice. You can easily select from preset frequency ranges for each of the 4 band graphic EQ which makes it quick to edit. I think this is a really good design.
Despite these limits, the cab sim sounds pretty good. I had no trouble getting some good results quickly. It even has recommended mic type and axis setting for each cabinet and I think following those gets you pretty far.
Some small graphical blunders aside, I think Blackstar has done a good job with the visuals and especially usability here. It's immediately intuitive to figure out the signal flow, switching sections (pre-fx/amp/cab/post-fx/eq) on/off is easy as is navigating and adjusting everything. It even lets you adjust the amp sim when on the cab sim page, which makes complete sense. IK Multimedia should look at this for pointers.
I think this serves its purpose well: It will get people interested in the real amps. If I find a St. James EL34 to try, I'll give it a go. Bundling the plugin with the amp would be a great move for Blackstar to bring added value.
Overall it's good value for money as long as you like how these two amps and the assortment of cabs sound. Certainly no worse than anything NeuralDSP or ML Sound Lab has put out.