I just stumbled upon Disqus today and am wondering why I would ever need to create my own Comment/Post model again :). Does Disqus store comments in their own database or do you have to store them too? How does this impact performance?
Is Disqus worth it?
Disqus stores them and provides admin tools, etc.
The performance impact actually in the negative for your backend (since the comment stuff is loaded and inserted into the page at load time) but obviously not for your users :)
You can use the disqus api to get your comments and insert them into your page at generation time if you like, but the usual way is just to include their javascript.