993RS

tmc

I have tried something similar. I think they're all made in the same Chinese factory. Some work okay, but it's not as good as the tire pressure monitors that are inside the rim. The screw-on caps have two issues. One is that they look like lollipops because they are so large. The second is that in order to measure the pressure they have to depress the pin in the valve. That means that it is the threads of those caps that are keeping your wheels from losing air. In order to add air, you have to use the wrench to loosen the lock nut before you can unscrew the cap. Needless to say, the wrench becomes a very important items to keep in your glove box.

Churchill

I keep one of these in the glovebox.....much simpler than hokey TPMS add-on systems.

centerpunch

Some cars (including my '06 Mini Cooper) use an "indirect" system that tracks wheel rpm of each wheel using the abs sensors.

http://www.tirereview.com/indirect-tpms-imports/

Seems like it might be possible to make an aftermarket system that worked the same way.

element

the mini system is genius where it uses the ABS (i think) system to monitor if a wheel starts rotating faster (when pressure goes down) than the other wheels. I had a slow leak in my mini years ago and it caught it within a few PSI. i don't know why others don't use something like this to monitor pressures for flats!