I'm using CComments for article comments, but would also love to use it for reviews - I think this could be done so easily with a couple of minor adaptations (eg - a rating possibility in the custom field option).
Also - I'd really love authors of my K2 items to be notified of comments on their articles - is there no way to do this?
Believe me, I thought about ratings. It's easy and extremly hard to achieve - depending on which road you choose. If it's only showing few stars without any kind of sorting on them, then it's easy. But if you need sorting based on that feedback you can forget it with the current structure.
As far k2 is concerned - just select the option "make author of article, moderator for this article" - it's in the settings under security. This way the k2 authors will be moderators and they will receive a mail when someone posts.