I never heard of them until a couple of years ago on this newsgroup.
Fantastic, they are.

When my house was build, the baseboard heating tubing had to take a
bend at the shower stall. The plumber put in a bellows type of flex
unit and after a few years it started to leak. I replaced it and it
was a PITA because of the location, difficult to sweat the joints. A
few years later, it leaked again.

This time, I drilled a hole in the base of the shower enclosure and
ran a length of PEX to the other side, connecting under the stairs of
a split entry stair. Very low head space. Again, all was well for a
few years and the couplings leaked. Tightened them and all was well
or another year. This time the drip would not stop.

I bought a couple of Sharkbite couplings for 3/4" tubing. Cut a new
length of PEX as it had to be slightly longer. In the bathroom, it
was just snap/snap, then crawled through the closet and under the
stairs, snap/snap and I was done. Wish I knew about these 20 years
ago when all the trouble started.

While they are much more expensive than regular couplings, they have a
place where they save a lot of time and trouble.