I also considered the Platinum Preppy highlighter, which appears to be rather utilitarian and environmentally friendly with its replaceable ink cartridges (just like the fountain pens) and felt tip refills. That said, I'm not a fan of the Preppy fountain pens, and I assume that the plastic cap and/or body of the Preppy highlighter is made of the same cheap and crack-prone material as the Preppy fountain pen. For that reason, I passed on the Preppy highlighter.
I stumbled upon the Pentel 24/7 highlighter while drooling over the fountain pen selection at JetPens. I thought it looked cool and felt that it would spruce up my office space if left on my desk, so I bit the bullet and spent $1.10 to acquire one.
The 24/7 highlighter has a section that resembles the section of a fountain pen, and it features a transparent cap that includes a spring-loaded tip seal. I admit that the eye candy aspect of the Pentel 24/7 played a role in my acquisition decision - I'm a shallow person.
I would say that the 24/7 is a semi-demonstrator pen because it has some transparent/translucent features. Moreover, you can see the liquid highlighter ink sloshing around in the reservoir chamber. The plastic label covers most of the ink chamber, which is a shame because it would be nice to see exactly how much ink remains. Conspiracy theorists may assume that the label is used to obscure the fact that the ink reservoir is much smaller than it appears. This could be true, but what do you expect for a buck ten?
The standard issue highlighters in my office are the big fat Sharpie Accent markers. In comparison, the tip of the 24/7 pen is much finer and more compact. This could be a plus or a minus depending upon your highlighting task, the font size you are dealing with, etc. Personally, I wish the tip were slightly larger, but whatever. I will find a way to deal with it.
Update (April 26, 2013): the ink volume is disturbingly low in this highlighter, and it runs out of ink too fast. I've gone through three of them in no time. I guess that's why they are so cheap.