I purchased the CometChat block in the morning. Installed it and it did not work. Contacted CometChat (via email) and Concrete 5 (via support form) both at the same time to fix the problem. I gave them my login info for both my hosting account and concrete 5 account. Went out for the day and then came back that late afternoon and it was fixed. Concrete 5 fixed it. Received and email from CometChat saying that I needed to contact Concrete 5 to fix it.
Everything works great! I see some questions on this board about what functionality it has. ideasponge below is correct. It is a stripped down model. Just has the Private Message app and the home icon pointed at cometchat. But, it works for me...that's all I wanted.
So, overall I give this app 4 stars. It would have gotten 5 stars if it worked when I installed it. But I was very impressed with the customer support of concrete 5. Good job guys! I now have a warm fuzzy feeling that if I ever have a problem with one of their apps in the future they will fix it.

I, admin, want to use Comet Chat with forum moderators only, to chat about trolls and private hush-hush forum stuff. I don't want regular users to be able to access it at this point, only members of admin and forum moderators groups(which I've set up) adn perhaps other privelidged groups with whom I might need to communicate. Does Concrete5 Comet Chat integrate with usergroups?

And if I'm already here, I see that TinyMCE editor is commented out of the Discussion forums code by default (which I've already bought and like a lot). Why is that?

I've never thought much of trying to pull of chat with JavaScript, but this app seems to do a darn fine job of it.