I personally found Notebooks extremely useful. Yahoo had a notebook feature and, when Yahoo Mail was my main email application and Yahoo was the primary platform I used, Yahoo notebooks was a place where I kept a lot of information. When the time came that I decided to migrate everything over to Gmail and Google, notebooks was one of the services I was concerned about losing. I was delighted when I discovered that Google had a notebook service, too, and in fact, it was even better (of course!). The ability to share notebooks was quite useful.
Now, as Google has decided to move away from Notebooks (effectively) since much of the functionality is available in other services, e.g., Google Documents, I finally got around to moving all of my notebooks off. I deleted some of them and migrated over 34 to Google Documents. I don't think the sharing came over, i.e., I'd have to re-share any of them. That's not so bad since none of the sharing was critical. Most of my shared notebooks were shared to a Google userid that I don't really use on a daily basis any more. Some others were for specific activities and projects that have ended.
So Google documents is adequate for the task and may even have some advantages. Still, I did enjoy the Notebooks app.