We (News360) were one of the launch partners for the OneNote API as well and our experience with it has also been really positive - the API itself is pretty sim

We (News360) were one of the launch partners for the OneNote API as well and our experience with it has also been really positive - the API itself is pretty simple right now but has all the details right. It's a great foundation to build on, and the team building it seems really committed to keeping the momentum going. Having external tools, both hardware and software, is a huge part of building a memory-augmenting product and I'm really happy that Microsoft is getting it right - OneNote has always been a really interesting product, but it suffered from weird positioning and always felt like an add-on to Office more than anything else. Now that it's being marketed as more of a standalone (and free!) product, I think it has a great chance of becoming much more mainstream.