Just personal experience, but I bought aftermarket sensors from Tirerack and kept getting flakey signals. Plugged into a STAR and serial numbers were coming back odd...when you run a diagnostics, you see which tire has what sensor serial number as well as some extra historical pressure data. However, for two of the tires, it was registering the same serial number which tells me the car was picking up the sensor from one tire to two locations. Sensors should have a more minute range, but somehow signal from one was overpowering the other. Another thing is I would get intermittant cutout of signal. This means that while driving sometimes, signal would cease and car would throw the big red flashing alert warning. Later it'd pick up the signal again, but was super annoying. Ended up buying OEM TPMS and having to pay again for reinstall.
Maybe was just the set that I got, but still...