I find it to be unusable for projects of any decent size. There's pretty much zero visibility on what everybody else is doing, discussions are hard to use when they go over five comments (for example, you can't easily binary-search a large discussion with Home/End or my favorite vim-like navigation plugin, because you don't have the full page right away and it loads a few comments at a time). It's very JS-heavy and writes multiple gigabytes of data to localStorage every hour (which wears down my SSD needlessly) — the only web application I've ever seen doing that. I can go on.
Generically, we're talking "Kanban board" with swimlanes for the status, tags for categorization, and comments/attachments for discussion tracking.
I'm currently in a position to get some people sync'ed on projects for a client, and I am going to recommend we use Trello.
Kanban is just the right level of organization and visibility without going full agile-daily-standup-sprint overload.