i had to send one controller back to sony due to rubber wearing away and splitting, i have 2 controllers and now either the replacement or the other (cannot tell which one it is) now has some slight wear and tear on the rubber, waiting for it to fully split though.
i purchased 20 thumbsticks off ebay and a technicians screwdriver set off amazon though, total cost about £20 but at postage of £3 odd each time and being without a controller for up to 2-3 weeks at a time, i'd rather fix them myself.
i may wait until 11 months after launch and then send both back with faulty sticks to hopefully get a newer revision of the DS4, you would think that they would have fixed it by then.