The TPMS earned its keep for me today. Got in this morning and got a warning from the system that there was a pressure imbalance, with the NSF being about 4psi lower than the rest. Checked it and it was, so pumped it up but continued to lose about 4-5psi during the afternoon so just back from the tyre shop. Huge steel tack through the tread, easily repaired at little cost fortunately. The TPMS did its job alerting me early so its fixed today, otherwise I suspect I wouldnt even have noticed until at least tomorrow as there was little visual and no driveability warning of the lower , reducing pressure. The absolute pressures, as others have indicated, are incorrect (all about 3-4psi lower than a good pressure gauge) but the relative pressures seem to be fairly accurate.