The best thing in the world is having people use your library and depend on its functionality.
The worst thing in the world is to have people that have more pull than you start dictating what should go into the library.
I get a warm fuzzy when I learn that someone else is going to use an application or library I wrote for their next project. It is nice to know that all my work wasn't an academic experiment, and I love knowing that someone found a real usage for my ideas.
I get very depressed when I am required to implement something that I feel doesn't belong in my library. Worst, the functionality typically isn't fully baked, and no one really knows what they want. Cruft and spagetti code soon ensue as modifications and changes are made in order to complete a short term requirement instead of a long term architecture.
... but architecture design is for another post.