All new cars in the EU must now have them.
Generally they are more trouble than help.
The early systems were too sensitive, often shouting at you with only slight pressure variations, often due to normal driving.
To avoid these false alarms, most systems are too far the opposite way, only warning too late. Often you'll hear the pop, feel the tyre deflate, pull to a stop, then the warning comes on. They are only really of use if the tyre is deflating gently over a few miles.
Aftermarket systems can vary between ok to absolute rubbish. Probably not worth the hassle. Some place sensors inside the wheel, some are part of the valve, inside, so both require tyre removal and rebalancing. If part of the valve, care is needed when buying new tyres not to lose the sensors.
These systems are not a replacement for daily visual checks and a fortnightly proper pressure check.