I love how easy it is to use, you can create boards in minutes with no technical knowledge required! The interface could be better but that's just nitpicking at this point - everything works great as-is so far (and we haven't even used all of its features yet).
We're using kanbannail mail primarily because our team has been able quickly get up an running without any training or support from us; also makes collaboration easier than other tools since everyone gets their own copy/board instead one shared space like Slack etc..