I'm one of the developers. Would love to hear people's feedback. We're trying to create an automatic but fun way to stay connected, and building some interesting contextual awareness along the way, e.g. https://steventheapp.com/e/Q1fJnL/subway
Why emoji and not arbitrary icons?
If they were using their own icon set I'd probably be asking the same question in reverse.
Emojis are becoming a very well known icon set among loads of different people, and I think it's smart when people capitalise on that to enable new things.