So if you listen to, for instance the 8dio demos of their solo strings, they sound really good - and they're just not (I maintain that the most productive thing to do with the 8dio Anthology/ Adagio solo strings is to delete them from your disk and never think of them again.). So you're right to be sceptical of demos.
The JB though - it does exactly what it says on the tin. It really is that good.
To really see this - check out Alex's live stream that he did on release - where he plays it live, and answers questions for I think a couple of hours. That will give you a very clear sense.
It does have it's limitations of course. It doesn't have dynamics crossfade like, for instance, the Spitfire solo strings, which restricts your ability to craft arcs. Similarly, it doesn't give you the kind of extensive idiomatic content that you get in the Emotional Violin. And so on.
And the JS demos are of course chosen to fit in the sweet spot of the instrument - so you can find lines that the JB isn't design to play well.
(If you're interested in the comparison - I go on about it at some length on this thread:
https://vi-control.net/community/th...-joshua-bell-and-spitfire-solo-strings.87635/
)
But within the expressive space it is deigned to reach - yes the JB really is that good. Especially the pristinely smooth legatos. There's something like 13 sampled legatos here, and there is nothing comparable to the smoothness and range and versatility of the legatos in the sampling world. Except that, kind of hilariously, during the live cast, Alex seemed to periodically remembering more types of legato that he had sampled, so who knows.