[Gaia]

It's okay... I listen to it from time to time. Her voice is quite cute. I believe it's meant to sound like an android girl. To be honest... I'd never be able to listen it non stop. It starts to annoy me after a while.
Japanese people treat her like a "virtual idol". There's already so much fan art. It's comparable with the Touhou universe. It started with a basis made by ZUN, and the community created everything around it. This principle is making it even more addictive for the "otaku community". The possibility of self-creation.
Miku Hatsune is/was a good way for doujin music artist to have a vocal for their doujin albums. It's luxury to have a singer in doujins, because the good singers will only go to the well-known doujin music labels. That's why we had a lot of vocaloid doujins on C73.
Anyway... I believe she'll be forgotten or replaced by a another vocaloid software, once people got enough of it.