Platinum Preppy Ink Tip Refillable Sign Marker Pen - 5 Color Set

Excellent pens. Best of all, you can buy rubber o-rings and convert these sign pens to accept ink such as Noodler's (see the jetpens article on converting the platinum preppy fountain pens to eye dropper fountain pens - the same o-ring size works on the sign pens). The converted pens produce a great look when you use the electric/highlighter inks (I've tried Lightning Blue, St. Patty's Eire, Hellfire and Year of the Golden Pig). The writing also looks good with regular Noodler's ink (I've tried Ottoman Azure and Gruene Cactus). Since the felt tips are replaceable, the ink options are limitless. Great pen, the tip is fairly fine for a sign pen, and the transparent body allows you to see how much ink you have left... and it's a beautiful pen!

Fantastic as an eyedropper
I did not try the included cartridge. Instead I converted it to an eyedropper and loaded it up with Dr. Ph. Martin;s Black Star HiCard.

I like this combo a lot better than the Sharpies provide at our office for Agile Story Card planning. It makes a bold but crisp line. No wear yet on the tip.

This is just an update and idea-
I took to traveling to class with this pen, and it leaks ink similarly to a fountain pen, such as if you move it quickly or drop it accidentally, then ink will make a Jackson Pollock painting inside the cap or on another hapless object.
A great use of this pen is to take a razor knife and modify the replaceable nib to have more varying angles and widths. (think of a quill pen) This makes the pen much more comfortable and expressive to use.

I did really like the design and performance of Preppy sign pens. In fact, they appear to work very well with converters and Noodler's ink.

However, there's a serious problem that plagues them: the plastic enclosure is quite flimsy. One of the pens I have have already cracked, even though I was especially careful not to apply any excessive pressure.

This is a good pen with a nice, vibrant color. I like that it can be refilled with a color of my choice. The only fault I find is the tip being slightly scratchy.