There are a number of clear benefits in this system over Disqus (which I currently use on this website), or a "roll your own" alternative, such as:
Right now I am planning to stick with Disqus because I like it's simple and elegant interface, however it will be interested to see if the larger sites out there that are currently using Disqus will make the switch to Facebook in the months ahead.
Wired have a good overview of the new system from Facebook here: Singel-Minded: Facebook Comments Are Another ‘Good News, Bad News’ Proposition
You can view a video from Facebook of the commenting system in action here: Improved Comments on Other Sites
Finally, here is the official page from Facebook on their comments system: Comments