I don’t see Google Notebook as a del.icio.us competitor. They have Google Bookmarks for that, and in that comparison, del.icio.us is the clear winner.
Where I see Google Notebook being useful is an online, cross-platform, always available version of Microsoft OneNote. Instead of clipping things and saving them to OneNote, you clip them and send them to Google Notebook. Store interesting bits, images, and (hey!) notes, and always have them available when you’re online (attractive to people like me who live in Firefox, but work from several different computers).
Anyone who uses OneNote for this kind of thing probably already knows what I’m talking about – if you don’t, give Google Notebook a try. They’re very similar in their behavior and features (with obvious differences, too). There’s also some wiki DNA in there somewhere.
Now, how about getting ink support for Tablet PCs enabled in Google Notebook (and Firefox in general)? Then we’ll be talking about a OneNote killer.