Based on the intro video on their website and the linked article, it's an LLM agent for your Mac, so in short, what Apple Intelligence should've been from the very start. Create calendar events, look up information, send messages, etc.

In the showcase video on their site I saw it generate an AI reply to friend group.

  A.C.K.: Wanna get [drinks] before?
  
  Sky: Drinks at Trick Dog before dinner work for you guys?

I can also imagine it being really good at getting a high valuation and selling for millions/billions of dollars.

But does it do it well, or are we focusing on how nice the Mac UI is?

It does just one thing really. It bridges the gap between what you want to do and how to do it.