I just want to say that I have been moaning and groaning my way through a project right now, using Overture with GPO. Turns out that it takes quite a bit of futzing around to get the sound playback to sound anything close to what i'm trying to notate. I am sure if I was trying to produce a polished mockup, I would prefer to work in sonar anyway. When I'm composing I just want to hear a good enough approximation. IMHO, in an effort to be flexible, Overture provides too many things to have to tweak, just to even hear a basic rendition to give me an idea of the colors I'm creating. I have the same issue with Finale Human playback, though perhaps a little more finely tuned due to Gary Garritan's involvement with the Finale Human playback features of finale. Of course Finale is too complicated for other reasons.
I purchased Notion and and uninstalled it rather quicly because i felt the editing work flow was just really cumbersome compared to overture. I hope that is being improved. What now interests me most about Notion is that perhaps I will be able to get this "good enough" sound playback while I am composing my ideas... I pretty much will have to use Finale or overture for final printing and I would probably use Sonar for mocking up something that sounds really great (using my Kirk Hunter libs most likely).
One suggestion I have is to offer some bundle deals for the sound expansions. For example, but the expanded strings and solo strings together and get a discount somehow.
I hope the editing work flow in Notion will get to where I hope it can be soon...then I could easily see Notion becoming my main tool for the composing and arranging step, particularly if it exports MusicXML that is very Finale-friendly.