Writing wouldn't exist without pens...
A good pen feels comfortable in your hand, won't make your hand cramp-up if you write for a long time, flows beautifully across the page without snagging or blotching and makes you happy to write. Well, that's what I want from a pen!
Of course, how your pen works is hugely dependent on the paper you're writing on too. I love to write in fountain pen, but only if the page is smooth and doesn't snag the nib; that there isn't feathering, or bleed-through. If the paper makes the pen or the writing play up, it ruins the flow and anhiliates my enjoyment of writing, and despite my love of FP and ink, I will reach for one of my reliable Parker ball-points (I have one in each filofax and several in my desk!).
I have on my desk right now three fountain pens, with another few knocking about in my desk. My favourite to write with is probably an Osmiroid flat-tip medium nib which I must have had for about 20 years and still adore. I'm currently on blue-black Quink in it. I'm not a lover of washable blue ink as its insipid colour makes it look like it's not quite sure about being an ink and is keeping its options open. Black or blue-black have the courage of their convictions and hell mend you if you get them on your clothes! I like that!
I also own a few calligraphy pens and nibs, but for the most part these are hobby/treat pens rather than something I use every day.
I don't care much for biros - I find the writing tips just a bit too pointy and 'diggy-in'. That said, I've managed to half-inch a few from people and they are okay. It's mostly the ones in hexagonal cases I dislike. Maybe I just don't really like cheap biros??!!
Gel-pens are something that I like the sound of then don't enjoy using. They are smoother and slicker across the paper than a biro or a ball-point, but with most of the disadvantages of bleed-through on various papers and the 'diggy-in' feel of the tip. However, my out and out least favourite has to be a fibre-tip pen. I see no advantages to them and lots of disadvantages - they scratch on anything but the most expensive paper, bleed-though like crazy and the points wear down into little stubs. Yuck. Yuck, yuck, yuck.
Although the blog (and this post) talks about pens, I have to say, I have no qualms about using pencil and always have a pencil tucked into the other filofax loop. I don't know why so many people have hang-ups about adults using pencils. I find the writing is smooth and straightforward (and erasable!). I don't much care for hexagonally shaped pencils as they dig into my fingers - I like the wood to be unpainted, smooth and round.
No, give me a fountain pen, some sticks-to-its-principles ink and some fine paper and I'm a happy lass. I'd be interested in what others use though. Does it relate to the writing you're doing or your job or is there a deeper 'feeling' also dictating what you write with?