The user also gets a nice page were she can access all her comments made on Disqus-enabled sites, can log into her account using OpenID and can rate other comments as well as get notified on new comments via email.
From the site-maintainer's perspective this service offers a simple way to integrate comments, offering SPAM protection and centralized moderation, rating of comments and so on. If you want to, you can also access all your site's comments through a currently read-only API, which means that you won't lose your comments if you want to leave Disqus again later on. There is even a separate documentation page for all the IDs and classes available for styling.
The system is very easy to integrate (basically all you need is a small HTML-snippet) and also allows testing from localhost. On the other hand you might run into some small problem if you for instance have your comments on a separated page (like here). This is quite easy to resolve, however, but just specifying the "correct"
disqus_url wherever you are:
@@ html @@ @@
This seems to override whatever URL got auto-detected. On the other hand I haven't yet found an efficient way to get the number of comments for not one post but multiple through the API, which is useful for archive pages or index pages on a weblog. So far I could only find calls for single URLs.
There is also currently no way to search your comments or if you are a webmaster to import your old comments. I guess the latter is a little harder to solve here since ideally you'd have to match e-mail addresses with current Disqus accounts.
I guess, in the end I'm still torn. Disqus' feature-set is really nice, but it's really quite a different way for handling comments that I'm used to. With Disqus I'd still be in control of who can comment on my blog, just not 100% anymore ...