I think I’ve found the killer features of Safari Books Online, and they’re two of the most obvious, for once. They are the bookmarks and notes features. Bookmarks are great for keeping your place in the book or tracking a very few places of interest in a book. They are not terribly fine-grained, though — they mark specific chapters, rather than pages. Notes have the same resolution but give the ability to store a note with a title and about a quarter of a KB of text. So far I’ve used this functionality to make longer-term, more specific bookmarks and to note where I see blog fodder.
It’s funny how two tiny features with only the bare minimum of integration can push the relative worth of the subscription from “worth it” to “addicted”.