I've started using GPO4 a little more intensely now and I can say that it's definately not a smooth ride.
Putting that a side for a moment, I have a couple of suggestions:
1. Enable draging of instruments from one ARIA slot to another along with its EQ, reverb and other settings instead of needing to set up the instrument from scratch.
2. Have a mark in the instrument list next to the instrument currently loaded into a slot. The way it is now is that after I load an instrument and open up the instrument list again, part of the instrument name is cut off by the instrument list. This and the ability to quickly find the currently loaded instrument in the list will be a good time saver. The mark should be placed next to each sub category in the list and next to the instruemnt itself in the last sub-category. That way I can navigate quickly to where the instrument is in the list and locate it quickly within the last sub-category.
3. I'm sometimes getting phasing problems with the string section instruments (not solo instruments but the section ones).
4. Add meters to the ARIA player so that we know which slot is playing when and how loud etc. Right now I have a piece playing but while looking at the ARIA player I have no way of knowing which of the slots is playing when and how loud (is it peaking?).
As I said, my first steps with the ARIA player and GPO4 in general not too smooth. Hopefully my comments can improve the user experience with the software.