I do use it as well, it seems to be quite popular with non-US mobile users as well given the lack of free, unlimited SMS in other parts of the world.
That said, I would say my biggest feedback item is that the UI is a bit busy. Don't get me wrong, functionally it is great and I like the fact that it uses your mobile number to locate you.
But if you could make the UI much more clean and modern, I think I would tend to use it more often. A simple background (white) with simple chat bubbles and less clutter in the message screen would make it much more visually appealing, IMVHO.
Call me an aesthete, but that's the only thing keeping me from using it more frequently.
But I definitely offer my congratulations on building such a popular and widely adopted app. It's not easy to get that level of adoption! ^^