What does Evernote do well? Evernote is a great way to organize notes. Your account saves/syncs across devices so you can easily access notes across your work computer, cellphone, etc. You can even share notes with other people with a few clicks. The interface is simple and easy to understand. It does what you would want a notebook to do.
What does Evernote not do well? Nothing that I can think of - I've only ever used a free version and it has offered everything that I need to store notes at work.
What do you wish you had known about Evernote before using? The ability to create shared notes with other people - I only discovered this after using Evernote for quite some time.
Have you used a competitor to Evernote? If so, how does it compare? Used to just use the Notes feature on my Macbook. Evernote offers way more features and allows me to access notes on whatever device I'm using (not just my Macbook).