What does Trello do well? Trello is great for organizing. Boards are easy to create and share with coworkers. There's no limitation for creating the board you wish, your only obstacle is your imagination. The use of private boards is something great also, This is really useful if you need to maintain private information.
What does Trello not do well? The private board policy is something that changed in the last months. Before they gave x amount of private boards, but I understand is a business and they need to get some money.
What do you wish you had known about Trello before using? The sticker packages for displaying in the cards are generally pay features. That's something they don't really promote and causes people confusions. Sometimes you're trying to activate and until you read the tooltip you discover is something you have to pay.
Have you used a competitor to Trello? If so, how does it compare? I have use trello since 2013 and haven't found any really competitor. It's easy to use and you can share it easy with your coworkers. Most of the companies I have worked used this for simulating the scrum board.