Speaking of Dark Matter
Intense Debate just did another release and wrote up some of the features in their post Comment System Makeover. They are on a regular release rhythm (about twice a month) and pushing a lot of new features and performance enhancements.
In an effort to be recursive, please leave me comments on your experience with Intense Debate as a user as they are looking for as much feedback as they can get (both good and bad is helpful.) And – if you have a blog, while you are adding Lijit as your search capability, give Intense Debate a try as your comment replacement system.