FHS Racing makes decent fittings, but yes, they are $150+ ea. for -6AN sizing.
The only place on a car that you should be using this type of fitting is:
1. where you *need* ~180 degree movement or multiple positioning without tools (I'll give an example below) or...
2. where you cannot properly stress relieve a line that is in constant motion, such as a line traveling between suspension and body, or transmission and body/radiator
In the latter scenario, this fitting is still not a substitute for properly plumbing the system - it is a Band-Aid.
I would never run fuel through these fittings, personally.
As far as an example of a valid use for these fittings, I personally run a transmission cooler through the radiator but have my lines set-up in a way that allows me to quick disconnect the cooler (via dry breaks), swivel one transmission side line 180 degrees, and connect back to the other transmission side fitting (eliminating the cooler circuit). This allows me to pull the transmission without making a mess, or in a pinch, keep the car running if for some reason the transmission cooling circuit was damaged or leaking.
Here's some images: