As a teacher, I use colored pens all of the time, and now that I have found a smooth-writing, easy-to-erase product that comes in vibrant colors, I will never use other pens again. I will agree with others that these pens go through ink very quickly, but their effectiveness as a writing tool is worth it for me. I have also found that the finer tipped versions use less ink. And though refills are not available everywhere, you can buy them through Japanese import retailers. Pilot also makes a style of this pen with a barrel that feels more like a pencil and with an eraser on the cap; however, they are not refillable.
Pros:
-- Great color selection.
-- The pens write smoothly without skipping, especially once they're "warmed up."
-- The ink erases thoroughly (and I've never had problems with it tearing paper.
-- There are refills available.
Cons:
-- The ink is heat sensitive, so it is not recommended for important documents or those that will be exposed to extreme temperatures.
-- The ink runs out fairly quickly, but then again, it actually feels great to use up a pen refill (as opposed to all those partially used pens all over my house and classroom).
-- The pen has a design flaw in that the eraser is on the end of the pen. Since this is where the cap is stored when the pen is being used, the cap has to be removed to access the eraser.