I've been using Circus Ponies Notebook since 2004, so a great fan overall.

One trouble with the Spotlight support is that when you search for an item in Spotlight, it finds the appropriate notebook, but then when you launch that item, it just takes you into that notebook in its last saved state. Now to find the item, you have to go look at the index for the same thing.

Since I don't have everything in Notebook, I often need to go through this two step procedure.

Turns out OmniOutliner Pro has much of the same functionality and preloads and finds the search terms from a spotlight search.

Other differences are in feel, namely, that while Notebook has a cute notebooky feel, the UI in OOP seems more polished to me, as though more thought has been given to avoiding duplicate functionality in multiple ways, thus making it easier to remember how something was done.