In my dealings with notation, I usually find that no performer is ever going to be satisfied with the amount of notation that you put into your score. And since performers like to complain, this is something that you will be encountered with for the rest of your compositional life.
As for notation in compositions, there are lots of different examples that you can analyze. A good place to start would be with the music of Gyorgy Ligeti, such as in his lux aeterna. That piece sounds like he could write it using graphic notation, however, he writes the music out traditionally using very very precise notation that is down to the sixteenth note.
Going further, you could look at the music of composers such as George Crumb or, as was already mentioned, Penderecki's Therenody. That shows some good examples of abstract notation.
And then the music of R. Murray Schafer is a good example of artistic notation, where you might accidentaly think that what you're looking at is a piece of art, but as you look closer, you realize that it is meant to be played.
These are various influences that I have worked with in my own music in terms of different notational styles. I'll also post two links to two pieces that I have written that use different forms of notation.
This piece uses what I call proportional notation. This means that the distance separating the notes on the page is to be used as a cue for how long the note is to be held for.
Invisible for Solo Piano - sheet music
Invisible for Solo Piano - mp3
This piece uses experimental notation (again, my term, may not be what others call it) this piece has lots of lines in it that represent the music.
Intersections for 16-part mixed choir
I don't have a recording of this because the choir's I have approached say it's too difficult to perform. There is a lesson there, write with weird notation, but make it playable.
So, these are my thoughts on notation.