GREAT for little ones when waiting I got my 3 year old daughter one of these for Christmas. Our 6 year old and her were FIGHTING over who got to use it next. I didn't realize that it was REUSABLE and it truly is AMAZING. You just fill up the little tank in the marker with plain water and doodle away.
A couple things, though. Don't squeeze the marker too hard or the water leaks out. If you want the pictures to dry a little bit faster so you can use them again, it helps to wipe any excess water off with a paper towel or napkin.
The pictures look MUCH neater than the ones they had when we were kids (you know where they had only about 4 colors in the whole picture and they all sort of looked the same).
I don't think older kids would care for these too much, but I'd say for ages 2-4 or thereabouts these would be great. We took it along to the restaurant and it kept her happy while we waited for our order. the only thing is it takes about 5 minutes or so to dry, so you might want to have an extra or 2 if you want your child to be able to be busy continuously.
They're a bit expensive, but as far as I'm concerned WELL WORTH the money. If you happen to lose the marker, I've found that a stamp moistening bottle filled with water works just as well (although the tip is a bit larger).