I do use it. I'm in a few different chatrooms with friends that have decent traffic, including the general "hoon programming help" groups that have higher traffic.
It is pretty slow, both because it's a decentralized chat and thus things are hosted on normal peoples' home connections across the world, and also just because Urbit the actual binary isn't very well optimized. It's good enough for me, and I don't really expect it to be as good as Discord. I haven't actually hit any bugs in quite a while, actually.