Essentially, they take advantage of your RSS feed and formats it in a number of unique views, but only work in newer browsers.
There's different views depending on the theme of you blog. I like the one pictured above called "Sidebar" and have DNS mapped it to http://reader.wargamingtradecraft.com/ . It's basically a nicely formatted feed reader for just your blog.
Most of the others seem more suited for bloggers showing off their artwork. These are Flipcard, Mosaic and Snapshot. The final one is Timeslide, which I could see being used for news blogs that post a lot during a day.
A problem I can see with these is that if you direct enough traffic here, people could stop using your main page. Then, they miss out on all the extra features you build into your sidebars and any Pages you've added.