I just took a look at CometChat as a solution for my site. Initially I liked the service but now I am hesitant based on some issues I experienced in the demo, my realization the demo shows a $500 version, not the $50 version, and the issues shared in this thread. It would be a good idea for CometChat to provide 1 free license to XF for use on this site (assuming Kier and the gang would allow it) so that we could see the product in action and know first hand the answers to the many questions we have.
As for some of the questions in this thread, I located answers in the site's Q&A:
http://www.cometchat.com/faq
Do you use true Comet technology?
By default CometChat is set to use smart-polling i.e. CometChat will poll depending on user activity. If you activate our CometService, then CometChat will switch over to true Comet technology. No special hardware/software is required on your server and the true Comet technology will work even on shared servers.
What is your refund policy?
We can only provide refunds in the case that CometChat cannot be properly installed on your server. This means that both you and the CometChat staff are unable to install CometChat properly.
Are there any additional costs involved in using screen sharing plugin?
Our screen sharing plugin uses Adobe LiveCycle Collaboration Service in which you receive free quota of $15 every month.
What is CometService priced at?
CometService is available at following rates:
Upto 75,000 messages for $19/month
Upto 300,000 messagesfor $49/month
Upto 750,000 messages for $99/month