I had a fixed stock one once. They're a great gun. They do use a gas piston, so they shoot cleaner than an AR. The gas is adjustable for two settings, normal and full-blast. It takes standard M-16 mags. The lower is a cast unit, and the upper is stamped. It's the standard rifle of many countries, and has earned a reputation for being a strong rifle. There's really nothing wrong with the gun. The one I had was very accurate. It was a tad heavy compared to an SP-1, but not when compared to an HBAR. Parts are not as easy to get as AR parts, but more common than most people think. I see them for sale all the time. Most of the parts come from Indonesia, which produces the gun under license, but they're just as good as the FN ones.
The high price doesn't have much to do with rarity. Really, they aren't that rare either. The high price was caused by a SAR article that mentioned that registered drop-in auto sears were available for the FNC and you'd have a legal MG for under $2000 total. Once that hit the newsstands the price jumped from $1400 to $3500 really fast. It's gotten more realistic now, but they aren't the great deal they once were. Nothing to do about rarity, just to do about what the market will support. If the article hadn't come out, the gun would have stayed in the mid-high thousand range.
There's nothing really special about the gun, it's just a good gun. It's still a good way to go if you want a full-auto as well, just not the screaming deal it once was.
Ross