I had been wanting to get a few of these pens for a quite awhile and I finally picked myself up two of them a few weeks ago. I got the white one and the black one as it seemed like they would be the two that I would most likely want to put designs on with.
Overall, I am not that impressed with them. They are easy to use but require several coats for it to be opaque and not see through, I wish they applied thicker like normal nail polish. And if you don't put a top coat on right away (and apply it nice and thick), they wash off!
I feel like it's kind of like using Crayola markers or something to draw on your nails, just doesn't apply how I would like it to. I would rather use a skinny tipped brush like the L.A. Colors Art Deco nail polish. I have liked the results I have seen with that brand and I will have to try it sometime.
Pros - Easy to use (holds just like a pen), variety of colors, endless possibilities for designs. Check out this advertisement, for ideas!
Cons - Washes off if you miss any part of the top coat (which is good if you get some on your skin), hard to use with a non-dominant hand, have to apply several coats (i.e., you want to do a french manicure, apply white to the tips several times to make it white enough), kind of pricey (around $7 for one pen - watch for coupons you could use if you want to pick some up!)
Also, I don't know about other locations but in Southern California, these are constantly sold out at all the stores, the only colors that are ever left are ones like light pink or navy blue. Have you found this to be the same for you? What do you think of these nail pens?