What does OneNote do well? We use OneNote to take notes in classes, meetings, and about research ideas, and use these notes practically by sharing them with colleagues. I love the ability to organize and search notes easily, and to share these notes. This has been a huge help in group work and sharing notes with colleagues who are unable to attend certain meetings, or just to share materials with one another. I also love how nicely it integrates with Office and the windows platform in general.
What does OneNote not do well? The controls can be a little clunky sometimes and are often different from those of other Office programs, although, like all of Microsoft's Office programs, it's become more uniform with the rest of the suite in recent iterations (especially 2019).
What do you wish you had known about OneNote before using? OneNote comes with Windows, which is great, and there's also a(nother? maybe?) version that's included with some standalone versions of Office (e.g. Professional Plus). All notes sync fine across these, but it's weird to have two versions of the program on your computer, especially when the one included with the paid Office suite feels clunkier than the one that's built into Windows.
Have you used a competitor to OneNote? If so, how does it compare? Google's "Keep" is somewhat comparable, but honestly I tried it out and found that OneNote's Android presence is strong enough that I don't need Keep on my Android phone and prefer to use the same service across everything, especially since I use all Windows computers. I'm also aware of Evernote, which I think is the biggest competitor, but again, OneNote's integration into the platform I already use has kept me from trying too hard to use Evernote.