As much as I love digital technology, I still prefer using a whiteboard or paper for brainstorming and sketching out ideas. Obviously you can erase little errors quickly on the whiteboard, but I never really cared about having the ability to erase while using a pen and paper; I still remember the mess that erasable pens made when I was in college (admittedly that was a million years ago). Things have changed a lot since then. The 3-colored FriXion by Pilot that I have been trying uses a thermo-sensitive gel ink that disappears when erasing friction is applied (see the rough iPhone video I made to see another method for erasing).
The ink is not quite as strong as traditional ballpoint pens, but pretty close. The pen writes just as smoothly as a regular pen, however. The downside is if you leave your notebook in a hot car, for example, it's possible the ink could disappear (some people have reported this). And of course, you should never use this kind of pen for legal documents or writing checks, etc. These pens are very popular in Japan. You can read what people say about the pens — good and bad — here on Amazon to get a feel for what people think. I am still experimenting, but I really like the pens so far.
ABOVE Best use of a PC ever? Students use pens and post-it notes to brainstorm and organize their ideas long before opening up an app.