are flex PCB's really used?
As the comments suggest, the main use of flexible PCB techniques is interconnect: linking one (rigid) PCB to another. They can be produced in arbitrary shapes more easily and cheaply than thin cable looms.

Lots of laptops and tablets will have flexible PCB connectors for things like the screen.

Placing components on flexible PCBs has the problem that the components themselves are not flexible, so they tend to strain the joints. Unless a "stiffener" is attached behind the PCB.