We've been using it for a couple of months now and think it's a great tool to stay on top of how the team is feeling and how the product is progressing.
We like how simple it is. The slack plugin is awesome.
We would love to have a mobile app.
what makes it work is that it doesn't actually impose work on you. you just click-off how you feel and keep doing emails. In aggregate, it's creating information that no one person has.
Speed. I wouldn't deal with a daily update unless I could do it within seconds. The email fits inobtrusively into my daily startup routine.
I'm looking forward to 1-click (less than 2), but I'm sure it's coming.