I don't use Google Notebook too much (I prefer text files), but I've noticed that this small tool has improved the search feature. If you had searched your notebooks until now, you would've found only the notebooks that contain your search terms. Now you can find all the notes that match your keywords.
It would be more useful if you could see the highlighted keywords in the notebook too and if the search allowed advanced operators. When it was launched, Google Notebook didn't have a search feature for public notebooks.
Related:
10 feature requests for Google Notebook
Remove 'Note this' from Google search results