In my previous vocation my work entailed lots of meetings - big meetings, small meetings, one-on-one meetings, meetings, meetings, meetings. I took lots of notes and I found a diary to be invaluable - especially for reconstructing conversations at much later dates. I used fountain pens for the dozen years I was in that particular role - they helped make the meetings tolerable and the writing fun.
In my current vocation as a minister I don't have but a few meetings to attend and I still use fountain pens for keeping notes. I tried for a year or so to write my sermon notes by hand and then type them into either notes or a manuscript. I found that process to be cumbersome and now I do all my sermonizing on the computer; it is simply easier. I do jot notes from reading material in the preparation.
Last fall I started seminary and now I use my fps for lots of note taking as I'm reading. I also use a preppy highlighter and Year of the Golden Pig ink for copious amounts of highlighting as I read; in fact, I have three of them filled all the time. I've found my Pilot VP is a great pen for reading notes - click it and write a few lines and then click it; works great.
As far as general writing goes, I use fp's 99.99% of the time. I carry two or three in my shirt pocket and four more in my bag. I've written exclusively with fp's for 17 years or so.