Haven't used it; but some of the demos sound interesting.
From my personal standpoint, probably one of my first
concerns is how well it integrates with notation software
(in my case, Finale).
Always a good idea to ask around, Fish... library demos,
of course, are intended to show the best side of a product
(and they should!) -- but how well a given product will
fit into your own work flow and tooling is also critically
As far as comparison to GPO -- I agree with you, we
already know the high quality of it. And of course, it
should never be a case of either/or. If a library brings
you the sounds and tools and capability you need, one
should certainly consider it!
I had serious problems getting it to work on my PC - severe noise issues and constant crashing. After several phone calls to customer service (which did not fix the problem), I uninstalled it and ended up giving it to a friend who uses it with DP on a Mac (more compatible).
The one thing I was able to discern in my brief time with it is that there didn't seem to be much in the way of velocity layering and that articulation availability was not consistent from instrument to instrument. One would have more difficulty using it as a "single-source" library for certain pieces that required more variety of articulations.
That said, the demos do sound very good, and if you're into historic instruments, it seems to be the only orchestral library that includes them.