Re: Notion,
Loading an instrument directlly with VSL sounds in Vienna Instruments (Pro) is easy and pretty straightforward. Loading a set of instruments hosted in Vienna Ensemble (Pro) is more cumbersome but doable. Notion's mixer is functional but rudimentary enough that you probably will want to use Vienna Ensemble (Pro) so you can avail yourself of it's mixer functions.
A user named Mike Meyers is partway through a series of YouTube videos showing how to creat a Vienna Ensemble (Pro) template in Notion 5, and there's been some activity around this subject on the official forum lately.
The Vienna Instruments (Pro) presets for the Special Editions (including PLUS) seem to work pretty well, and you can work with a sequencer version of each stave, though obviously functionality is not comparable to a full-blown DAW. You can customize your own Rules to control instrument behavior, and Notion 5 has made that easier than in the past. Still, custom rule sets are by nature pretty laborious.
Can't speak regarding Mac, I'm on a PC.
I agree with BIll, it's short on engraving, but if that's not important to you then who cares. For scoring and editing it is very intuitive and fast.
I have no personal history with the other notation programs. I will say, though, that the magnificent-sounding Steinberg project is proving to be a VERY slow, deliberate process. Don't look for an actual product any time soon.