What does the software or service do well? Stability and the fact you can self-host the platform, not just because of the cost, but because of security (hiding it behind a VPN and all that good stuff). Having bunch of plugins doesn't hurt.
What does the software or service not do well? Sometimes it gets too granular, but to this date we haven't found a thing which we couldn't tweak or add/remove, and adapt Redmine to our needs.
What do you wish you had known about the software or service before using? Ideally there would be a better and more supported eco-system of plugins, with reviews or screencasts or tutorials. Redmine has that "open-source community" vibe, which is great, but leaves newcomers wondering if it's "here to stay" or not. But it's a well established and proven solutions, do I'd definitely recommend it.