I love this plugin because I can write simple notes and have them display at the point where the client needs them. I also encourage clients to write there own notes so they have them for next time.
I first used it on a site with custom fields. However now I am finding more and more handy spots to use them, and when clients ask me about something, I have started adding the site notes, so that next time they ask the same question I can refer them to the site notes.
This means when you do things that a slightly non standard you can also write notes for yourself so when you look again you will know what you did 18 months ago.