I spent a few minutes with it today and I can confirm that the bookmarklet works with most pages in Firefox. It takes you to a page on the Readsy website and shows you the window there, and once you're done you'll need t click the back button to return to whichever page you were viewing before.
That's not a terribly good design (a popup window would be better) but it is enough to give you a taste of how the Spritz tech works.
Based on the demo video I found on Youtube, Readsy was released some time in May. It doesn't appear to have gotten much attention just yet.
According to the website, Readsy is the work of a sophomore at Princeton University majoring in Computer Science. It ties into the API provided by Spritz to increase up your reading speed using a concept called rapid serial visual presentation (RSVP). It works by flashing a single word in front of you at a time, at speeds ranging from 250 wpm to 600 wpm. RSVP been around since the 1970s, and as I have pointed out before it has any number of deficiencies, including inducing headaches, poor retention and comprehension, etc.
If you don't want to use the Readsy bookmarklet, you can also find Spritz tech integrated into The Old Reader.