It's a good start, but I feel the UX could do with some refinement. Why is there no default notebook upon signup? Why isn't there a big fat "Add a note" button on the top of the list of notes? Why do I have to explicitly press save after editing a note? And why is is soe obscure icon of a floppy disk, instead of a button with the text "Save changes" on it? Etc.

All the functionality is there, but as I see it, an app like this is all about good UX and very little about the technical features.