From what I saw Cosmic has a lot of potential and looks pretty sleek too, right now I'm using KDE it's a great desktop, but now that I have a second monitor it randomly crashes on me, I think I'll switch to Cosmic when it reaches beta.
I've been using it as my main for months. Even as an Alpha, it's very stable. That being said, it's missing quite a few features that a lot of people would consider a requirement. So "ready" will heavily depend on your requirements