I have it and it works great! I also use the highlighters which work even better. You can highlight it your text books all you want then when you sell them back you can just erase away.
Cool thing is the ink disappears at around 140 degrees Fahrenheit, that's where the friction part comes in and will reappear around 26 degrees.
You can use a hair dryer or heat gun to disappear large amounts of writings, like in books.
You can even make secret notes with if you write it, then erase it with a hair dryer , give it to someone who then puts it in the freezer for a couple of minutes then wallah it's back. O.O I know am weird but that's just me.xd