What does Trello do well? Trello makes it easy for teams to collaborate on projects, and for individuals to get things done. The visual layout is also appealing to upload projects and move them into different boards and/or status. The drag and drop feature is popular for software in general, and I feel like they were one of the first to really utilize it. I find that designers love the layout best.
What does Trello not do well? Most of the people that I know who love Trello are designers. They like the visual nature of it. However, I don't find it to be an intuitive software, and as a copywriter, I don't like the fact that I can't format text (ex: bold, italics, etc), and I always seem to have trouble attaching files and things. I just find it a bit frustrating, personally. I only use it for projects when I have to, like when a client is already using it. Otherwise, it's just not my preference.
What do you wish you had known about Trello before using? Nothing. The designers at my old job wanted to use it for project management, so that was my introduction. I've since used it on another project or two, one with a client and one as a volunteer. I just don't love it.
Have you used a competitor to Trello? If so, how does it compare? I use and love Asana. I like the list layout rather than the boards, and I love the fact that I can format text. I use Asana personally and with my interns. And I recommend it all the time. I think a lot of features are similar in software like Asana and Trello, so it often comes down to personal preference.