Disclaimer: I work for Omni and am a long time OmniOutliner Pro user. That being said, I think there is a case for both apps.

Circus Ponies Notebook is a totally awesome Mac app and from everything I've seen if you liked version 2 then version 3 is going to knock your socks way way off. You may never find you socks. Ever. That far off.

I used to be a big OneNote user back in the dark days when i was stuck on a PC and Notebook is the closest thing to that kind of all-in-one information organizer that I've seen (but with a much nicer UI and user experience). I like DEVONThink as well but it doesn't have the neat stuff like sketching and quick charts that Notebook 3 will have. Plus, the Circus Ponies folks seem like good people.

Our OmniOutliner (and OmniOutliner Pro) app is much more about structuring ideas and information, and I think it matches up well with any general purpose information management tool like Circus Ponies Notebook, DEVONThink Pro, etc. I love to collect ideas and information in something like Notebook and then once I'm ready to develop the idea further into real writing I get to work in OmniOutliner Pro.

I'm hoping to get some more examples up on the Omni website with details on how you can use the app for writing, lists, structure documents, etc. In summary, it's a great general purpose structuring tool that matches up well with general purpose collection tools like Notebook.