I have just bought Notion and, as I've said in a previous post, this is an awesome tool for learning to orchestrate your work. It's playback is pretty good, but I would say that these scoring programs can never been quite as finely tuned as the results you will achieve with a proper midi sequencer.
If only because you can't manipulate very fine points of dynamic data (Expression, Velocity etc.) in a score editor.
I have tried the demo of Sibelius and I have to say that comparing the two programs I find Notion a little more accesible. Certainly its a REAL help to composers like myself who know articulations by sound .. but not by name .. so being able to hover the mouse over 'Sul Pont', (or similar), and have a help bubble explanation of that is extremely helpful. As I say .. Notion is REALLY good for teaching orchestration to people like myself.
Notion is a little bit fiddly in some cases .. and importing midi is difficult to get things set the way you want. I tried to import a string ensemble and found that there's no easy way to move the data around as I wanted to. And you can't seem to select in parts (Meaning you select whats on the page ... move the page browser bar over to the next part you want to highlight and include with your previous selected parts (usually by holding down shift .. or alt and highlighting the next batch). This just doesn't work .. which as you can imagine is a real pain if you have a lot of data you want to move around. And I can't figure out how to insert a bar at a specific point.
Well these are all teething problems .. most of which are probably down to me needing to learn the ins and outs of the system .. but some are probably down to the developers .. who need to implement these things to make it better .. and I'm sure they will in time.
But the playback I've experienced in Notion so far is pretty amazing. It's much better than what I thought it would be. I must admit I like the idea that the Notion SLE for VSL is built specifically with VSL in mind. And I hope they come out with a full featured score app for the whole VSL library. And for $89 .. to be honest I can't beat that .. I can use this app to score my music, and to learn at the same time. Way better than the $650 it will cost for Finale or Sibelius.
Guess it depends on your skill level. Notion has my vote.