In practically all of the marketing literature touting the virtues of a particular pen, the gold content of the nib will feature prominently. For those somewhat new to fountain pens, nibs of modern pens will typically come in one of the following "flavors":
1. Stainless steel
2. Gold plated -- A thin layer of gold over a steel nib
3. 14k, 18k, or 23k gold -- A solid gold nib with varying karatage.
Prices of pens typically reflect this order as well. Pens with steel nibs tend to be the cheapest, followed by gold plate, followed by gold -- with higher karatage usually selling for more than lower karatage. The key question addressed in this essay is what exactly you are getting for your money when buying a 23k nib as opposed to a steel one. That is, will the features of a gold nib impart superior writing performance compared to a steel nib?
The short answer is no.
Here's why: Almost all pens of reasonable quality (from the Waterman Phileas on up) have nibs that are tipped with a ball made from hard metals. Sometimes, these will be referred to as "iridium tipped" although iridium is now used less often as part of the tipping compound. It is this hard tip that is the part of the pen that is actually in contact with the writing surface. Obviously, the smoothness of the writing is going to strongly depend on the quality and workmanship of this tipping material. One thing that the smoothness of the writing is definitely not going to directly depend are the materials not in contact with the paper -- that is, the metals that make up the pen's nib. Thus, the argument that the gold content of a nib per se imparts a smoother writing experience is pure nonsense.
So why would one expect to associate superior writing experiences with gold nibs. One possible explanation is that the gold is a signal of the workmanship put into the tipping material. For instance, tips to gold nib pens might have smaller variances in the composition, size, and so on. Alternatively, the finishing process for these tips might be more carefully done and be machined to higher precision than tips on steel nib pens. Again, note that this has nothing to do with the gold content itself though. The problem with this story is that it does not seem to be borne out in practice. If anything, complaints and disappointments appear to be more common for high-end pens with gold nibs than for low-end steel nib pens. Part of this of course has to do with expectations, which are higher for a more expensive product. In practice, perceived smoothness seems to have more to do with the company manufacturing the pen than to the signal value of the nib.
A caveat is in order here -- at the very lowest end, there is no hard tipping material. Instead, tipping is done by rolling the steel tines back on themselves to form the tip. The tipping strategy leads to much faster wear of the tip (because steel is a lot softer and more abradable than the hard metals) and often these are not smooth writers. Examples of this technology include the Pilot Varsity, the Sheaffer Reacktor, and many low-end Esterbrook nibs.
The other argument made in favor of gold is that, due to the softness of the metal, gold nibs are more flexible and hence impart greater character to one's writing. This claim is used on a comparative basis as well. For instance, when Pelikan switched the nib on their M600 pen from 18k to 14k, many observed that the 14k nib was much stiffer and this was ascribed to the lower gold content in the 14k. While it is true that gold is a softer metal than steel, it is not an especially springy metal -- and it is the springiness that is reponsible for nib flex. As an extreme example, imagine a pen whose nib was made out of pure gold. While it would not take much pressure for the tines of this imaginary pen to splay apart, the lack of spring in the gold means the deformation would be permanent. Such a pen would be useless to write with.
So why bother with gold nibs at all? The answer is purely aesthetics. Gold nibs are desirable for the same reasons that gold jewelry is desirable, it's simply more attractive than its steel or gilt brethren. Moreover, most pens that have attractive fittings and nice design also tend to have gold nibs. Bottom line: Buy gold for looks, not because of anticipated performance differences.
One final gold myth: Buying gold nib pens as an "investment" for their gold content is silly as well. The market value of the gold content of most pens is negligible compared to their prices.