I've been playing with this library for a couple days. I've very impressed with the playability of it. Not too thrilled with the tremelo and trills playing methods, but meh. The primary modes are amazing, especially the solo modes.
That said, I find the thing that trips me up is that accidentally bumping a key can easily yield dead silence. It is not too bad playing at moderate speeds, but when I really get going it happens pretty easily. My thought is to borrow on ideas from monophonic synths (the trill mode does similar things, but doesn't have the character that I'm looking for.) More or less it is that when notes are still held, the last note played (before the one you just released) becomes active. This would make it easier to do grace notes as well. One step further, I think the portamento should be weighted by how short the note was. For example if it was very short you'd have almost no portamento, and if it was long you'd use the velocity from either the grace note or the note you're returning from. I suspect the former would work better. I think this would allow people to fast passages much more easily. Maybe this would be a seperate mode if it violated too much of the playing style you were going for.
Also a side note, using sustain to rebow does not work in the solo mode when you do a polyphonic chord. I.e. play 3 notes together, you get the chord, hold sustain and play them again and you get only one of the notes. I would really like to see this fixed if possible.