*Update 10/30/09 I'm changing my mind here. With Disqus's latest update I believe it is just as good or maybe even better. For one thing Disqus is more widespread which makes it by default more social. Also, Disqus has a webpage where all your comments are aggregated so that you can easily access them and which can be indexed. Most feed aggregators use Disqus, so that all of your comments will be automatically sucked into them, if you so choose. This gives your comments a bigger footprint. I used FriendFeed, and I don't think they accept JS-Kit yet. I assume the other aggregators are similar. Now with JS-Kit you can manually choose to send your comments to various social networks, but since its not automatic, its tiresome to individually select and choose each one. So usually you don't, and your comment remains in a "silo." Now, the advantage to JS-Kit is that it's smoother and real-time, and I assume they will add all the features that Disqus has in the future. But it all depends on how popular the engine gets for it to be a default for the feed aggregators, which may or may not happen. So as of this moment I would suggest that you go with Disqus, or at the very least, if you don't add it to your blog, get an account, so that you can use it to comment on other blogs, most of which use it, if they use anything at all.
It's a real-time commenting engine for your blog or website that also gives you the power to propagate your comments across your entire social graph. It turns a static, "block-like" commenting section into something that is more alive like a river. It keeps the discussion of a blog post, or other content, on the content page itself. And when the comment section comes alive more value is added to the original content itself. It also benefits all of the social networks because comments become additional content that's added to their site. Also, it encourages commenting because they are more recognized, responded to in real time and propagated, if you want, to all or some, whatever you choose, of your various other social graphs. The company, JS-Kit, totally rocks. I love their attitude. It's about innovation. This new system, "Echo", will be coming out in a couple weeks, but if you want it, be sure and go to their site and sign up now, because they will be sending out invitations, or opportunities to install it, on a first come, first serve basis, to those who send out a special tweet.