What does the software or service do well? OneNote is one of several different note-taking apps I use. It's clear and easy to understand, and it's handy to be able to have the option to take "written" notes when you're using a Surface-style touchscreen device. It's easy to search through notes for specific strings of text, something I do often. The Word-like interface makes it very simple to understand.
What does the software or service not do well? OneNote isn't as packed with features as something like Evernote, although that's not an issue for me as it does what I need it to do.
What do you wish you had known about the software or service before using? Really I just wish I'd used OneNote sooner. Many people get it as part of Office365 but not much is made of it.
Have you used a competitor to the software or service? If so, how does it compare? I use both Apple Notes and OneNote regularly, but for different purposes. There's not a huge amount to choose between them. I tend to use Apple for more random one-off notes, and OneNote for more structured notes that I regularly add to.