First of all, it just "feels" really cool to use. It looks great, and it's fast.
It's also powerful, flexible, and integrates with a lot of apps.
Improvements lately, like the new Email Management system which enables us to send newsletters to accounts easily, have been really good.
The only thing I'm missing so far is something like a team chat, or a channel per workspace so that teams can quickly communicate about more general stuff. If they add this, IMO they have a clear winner.