I like these pens with erasable ink and in playing with them my kids and I
discovered a few interesting things. When you rub the ink it heats up and
"disappears". In reality, the ink turns white. If you try this on dark paper
(black, for instance) the text will show up white after you erase it.
If you heat up your writing in some other way, say, by leaving it in a hot car
or holding the paper up against a hot light bulb, it will also disappear. So,
don't leave any important notes in your car in the sun. But, if you want your
notes back, you can put the paper in the freezer. After a while your text will
be back again, albeit not quite as dark as before.
My experiences were from about 2 years ago. It may be that the inks have
improved. So beware, if the permanence of what you write is important to you,
don't use this pen. Be weary of diplomats you hand it to you to sign treaties
and that sort of thing :-)