Most individuals using FSR sensors either homebrew their own daughter boards (?) from a trinket / tiny85 or are using the boards designed by JohnL.
Upto three sensors plug into the board and monitor the average resistance of the sensors. If resistance increases (weight is added), the board sends a signal to the printer controller just like if it was a normal end stop. If the weight remains, the board will update the average resistance after a few seconds and continue looking for trigger events.
I bought my kit from Ultibots (without mounts) and it has worked great with the RepRapFirmware maintained by DC42 on my Duet 0.8.5 printer board. It should also work with Repetier as it should simply register as a simple end stop for Z-min.
