In the early days of the service, I was underwhelmed by Mixx. It was a nice enough Digg clone, but Digg was still better in all areas, and not only because it has a huge user base.
However, lately Mixx has been really trying to bring some features that Digg lacks to the table; first, they introduced Techmeme-style story threading, and now they've added the possibility to push some stories to the front page very fast if needed.
This is a sign that they really know their stuff. It's a little discussed but quite apparent Digg problem: sometimes, an important story will show up everywhere else before it does on Digg simply because it usually takes 15-20 hours for a story to pop on the front page. Digg solves this problem partly with their algorithm, which takes velocity into account (n.b. the folks at Digg do not disclose the secrets of their ranking algorithm, therefore this info comes from my experience and anecdotal evidence). It means that if some story gets a lot of votes in a short period of time, it'll go to the front page faster; it happened, for example, when the first iPhone stories broke out.
However, Digg community is not always so excited about breaking news; if it's politics or Apple related, sure; but in some other areas the velocity is just not high enough.
[img src="" caption="" credit="" alt=""]
Mixx takes a different approach. They've introduced a new user, a "Super Mixxer," which gains the ability to tag a certain story as "breaking," which in turn will make the story appear in a special category, "Breaking News." Breaking news stories can become popular just like any other story, but it gains additional visibility since the Breaking News category is visible on Mixx's "Popular" page. As an additional check, one Super Mixxer is not enough; it takes two to Mixx a Breaking News story.