Some time ago, I was lamenting the number of highlighters I tend to kill throughout a semester in some of these classes that require a lot of reading. It seems such a waste to have to throw away the whole pen when it's drained.
Well, now I've discovered an alternative and am trying it out. Noodler's Ink makes a line of highlighter inks for fountain pens. I'm using their fluorescent yellow Firefly Ink (which looks yellow on the page but an unearthly green in the jar.) I've loaded it into a pen normally used for Italic-style calligraphy which has a nice, broad nib.
So far, it's working pretty well. The pen's not really designed to flow this much ink at once, so I'm still playing with the technique. I like the fact that I have more control over where the highlighting goes and can refill the pen when it's empty, but on the other hand the pen's a little long for the pen pockets in the backpack. I'll probably post an update once I've had more time to use it.
Update: follow-up post here.