Literally useless. 700 wpm was easy, but what's the point? When reading for entertainment I like to stop and appreciate the clever wordplay or beautiful exposition of something or what have you, and to re-read some things, and to stop and let my my imagination work.
When reading research it's similar, you want to control the speed of your reading and to give yourself time to fully understand what you just read, that's how you remember it in the future. Reading at a normal pace you can stop and think about the implications and different applications, so it both helps with memory and makes you think and understand it better, as opposed to speedreading where the concepts just flash through your mind and you can't stop to think about anything.
And I hate how they're selling it. "Read a 1000 page novel in 10 hours, what could you do with your spare time?!" as if entertainment is some sort of a competition. Hurr durr, what could you do if you fast-forwarded every movie you watched?
I still subvocalize at 700 WPM and I don't see why I'd want to get rid of that, helps make it coherent.