At Appleby College, we've been using Microsoft OneNote for a l.o.n.g time now ... and structured shared OneNote Binders for two full academic years now. Folks often ask how much time it takes for teachers to get accustomed to working with OneNote and shared documents in general. Since we have the advantage of a 1:1 tablet program and a background with the software, it's hard to be fair when describing how long it takes for teachers to become comfortable enough to use it in the classroom in front of students and to bend it to their pedagogical and administrative needs.
Well, we started using OneNote last week at the Park City Mathematics Institute (PCMI) to help with professional development creation. Of the 50 or so participants, only a handful had used it before and for the vast majority it was brand new to them. So far, they've spent 8 hours using OneNote and shared collaborative Word documents & PowerPoint presentations through OneDrive and the WebApp. I can definitively say that 8 hours, using a series of practical activities that only tangentially required them to interact with and add to these applications rather than "training" is how long it takes -- people are already taking their own initiative and trying different things with the WebApps and determining when and how to use the desktop applications with shared files. As well, people who have taken the additional step of using a pen within OneNote have commented on the power it provides them, in particular in a subject like mathematics.
There are a few issues with Macs but iPads (with all their limitations) are working fairly well although the browser implementation on the iPad does get a little wonky.
If you're planning on working with teachers through a PD course, at a conference or even in day-to-day school life, you should give OneNote and the OneDrive WebApps a try.