The preceeding are nice technical descriptions of the workings and care of a fountain pen. They miss, however, the most important aspect of a good fountain pen - what a sheer pleasure it is to write with - in fact, this is the only reason to use them (other than, perhaps, for decorative calligraphy).
Good tools make the job that you are working on easier, and the end result better. A good keyboard, one that you feel comfortable with, whatever your definition of that is, will allow you to write both better and faster. Faster because you feel more comfortable with it, because it functions, mechanically, as you would like, and better because you are not frustrated by your tools - you can spend all your energy thinking about the writing, unencumbered by the distraction of a keyboard that is sticking, or too small, or whatever.
There are times when one has to write on paper: tests; paperwork; random scribbles in margins; etc. And there are times when it is just nice to write using a fountain pen, for a letter, or perhaps a manuscript, or because for some, it is easier to get started thinking when writing with a pen. Either way, it seems logical to use the best tool for the job.
A good fountain pen will give you nice, smooth lines. It will require little, if any pressure against the paper, thus your hand will be more relaxed, and you will feel more comfortable. It will feel almost as though the pen is floating over the paper. And the result will generally be more ledgible than that from other types of pens. I lost my fountain pen for about three months and almost stopped writing as a result... it just didn't feel right, writing with a ballpoint pen.
A cheap fountain pen will do some of this - the $7 Parker from the drug store will be better than nothing, especially if you get a converter, so that you don't have to keep buying cartridges for it. But you will still have to press it against the paper to get a mark, and it will feel a bit rough.
A bit better pen, say, a $25 Waterman, will be nice... you will get the feel of what a fountain pen is supposed to be... but it just isn't quite right.
Spend a bit more, something in the $50-$75 range, and you will have a pen that you will never want to be without. One can spend more, of course, but beyond that point, a lot more money must be spent for very little additional return.
Choose a fountain pen because of the way that it feels in your hand and how it glides across the paper, not by how it looks. The Rotring 600 series look like a ugly large hunk of metal (in a good way), but feel perfect to me. Choose what works for you.
And treasure the way it works so nicely and feels so perfect.