This is an excellent extension, and works as advertised.
However, the options available in the administrative back-end to customize its behavior are limited. To modify the appearance of the chat-box, you have to modify the css files in the module directory (which isn't a big deal, in and of itself). Several style parameters are, however, hard-coded in the .js files (the vertical position of the chat box being one of them). Not an issue if you know what you're doing, of course, but I would have appreciated some styling options in the back-end.
On the other hand, I find the absence of an option to set the refresh behavior (rate, conditions) of the ajax calls the extension uses to check for users/messages a little more serious. On a site with only one user (the admin) connected, it sends requests every second or so. These are tiny, tiny requests, but I feel like I shouldn't need to modify files to restrict when and how often these ajax calls are made.
Nevertheless, this is a great piece of software, and well worth the purchase if it looks like what you need.