Spritz seems like a very interesting way to read quickly. It's the opposite of everything I've read (slowly) about speed reading, which focuses on using peripheral vision and not reading word-by-word. You're supposed to do things like move your eyes straight down the page, taking in whole lines at a time.
Interruptions seem like a big problem; interruptions that make me look away, or interruptions in my brain, where I might realize I've not been paying attention for some amount of time. Maybe they should have navigation buttons similar to video players, so you can skip backward 15 seconds at a time. I also do want to go back and review previous pages sometimes for reasons that have nothing to do with interruption, so I wouldn't want word-by-word to be the only way to view a text -- especially when reading nonfiction. I might event want it to work in a mode where you hold down the button on the side of your phone or tablet in order to move the words, and then have them automatically pause when you release. It feels like I'd want a lot of short breaks when reading in this style.
It should also be free software, but unfortunately I'm guessing it won't be. I hope someone will make a free software application along these lines -- the basics seem pretty basic.