Good info so far, that and the link to the library should take care of most of your questions. My experience is that with nibs, you tend to get what you pay for IN THE LONG RUN. Meaning, cheap nibs in less expensive kits have a higher percentage of kits with nibs that don't work well out of the box. The more you pay per kit, the fewer problem you should find, but some will probably still sneak in there.
For the most part, I don't find much difference between the bottom of the barrel nibs, the more midline parts, or the ones in the "high end kits" once you get it working. Some people like the long straight sections on the jr and classic size pens, some like the flared or stepped larger sections with the 6mm nibs in the full size kits. But overall, once you get them tuned and working, they all write about the same. That much is pretty well known, and short a few minor variations, not very controversial.
Now, for the opinion part. As for the upgrade nibs, for the most part the quality control is tighter, meaning that the tines are checked more closely to ensure that the iridium beads are aligned, so that it doesn't feel "scratchy" because you have an exposed edge dragging on the paper. It is also much less likely that there will be trash caught in the ink channel or roughness on the underside to mess up the capillary flow of ink to the tip. But again, once you get it installed, adjusted and working right, you won't feel a huge difference vs a well adjusted and polished kit nib. A steel nib is pretty much a steel nib, regardless of who made it or who's name is on it, once it is tuned. As for the expensive solid gold nibs, my somewhat more limited experience is that for $75 or more you should not find one with those type of problems out of the box. However, a stock gold nib is again, not going to feel much different than a well adjusted and polished steel nib. The big advantage of gold nibs is that they can be adjusted by a competent nibmeister to "flex" or open and close slightly depending on the pressure, making the line on the paper widen and narrow as you write. Some people can use this feature to create written art that is often quite amazing. Others like me just produce illegible scratchings that are wider in some spots and narrower in others. (shrug)
But from a sales perspective as a penmaker, if you can adjust a nib well enough to get it to work correctly, most customers won't care what it says on the nib. However, the small percentage of customers that buy fountain pens consistently often will not even look at a pen with a kit nib on it, regardless of how carefully it was adjusted, and how great it writes. They just don't want a pen with a kit nib on it. So, I try to have plenty of upgrade steel nibs onhand, so that don't cost me a sale. For the rare (for me anyway) customer that is willing to pay the extra $$$ for a solid gold nib, most of them usually already have a relationship with someone that knows how they want their pens to write, and will customize it to their tastes. I've only had a couple of people who wanted one and either wanted to leave it stock or asked me to have it modified for them.
Of Course, YMMV so have fun.
