I've painted a chalk board onto one of my walls (using the blackboard paint) and wanted to be able to write onto it without getting chalk everywhere. These pens are brilliant - no chalk going everywhere and broken bits of chalk. They are really easy to use, although watch out how quickly the ink will come out if you squeeze too hard. You simply give em a shake and a squeeze and start writing - it even looks like chalk. They have a sort of italic nib and are surprisingly nice to write with.
Then when you want to rub it off you just get a damp cloth and rub away
EDIT: Above is my original review, but having left some of the "chalk" on for quite a while it has been very stubborn to remove. Been clearing the words on my chalk board for ages with a damp cloth and cleaning spray (non bleach) and it is faint but still there (you can still read it). Not sure if it is because my chalk board is a wall painted with blackboard paint, or because I left the writing on there for so long - either way you would have thought it would be easier to remove. If you want to use for an outdoor sign it will probably be perfect!