Hi,
I thought I'd share a little...
So about a year ago I purchased an aftermarket tire pressure monitoring system from TireRack:
http://www.tirerack.com/accessories/detail.jsp?ID=55
(btw: if you look closely you'll notice that the pictures ahve been taken in a P'car)
My main reasons to get it were:
a) to be able to get tire pressure readings during A/X and DE w/o having to jump out of the car after each run and start fiddling with a pressure gauge
b) as security for daily driving
So how did it work out?
Installation was easy, I just waited until it was time to mount new tires (which happens quite frequently for me ;-) and my tire guy just switched the old valves with the new sensors.
Nice convenience factor for A/X, but useless for DE. You may ask why?
Well this specific monitor not only measures tire pressure but also temperature and will sound an alarm if either is too high or too low, well that's good - right?
Generally yes, but at the end of a DE session my front brake rotors are so hot that they trigger the "hot" alarm, which will not go away until the temperature reading goes back down, and you CAN'T swhitch to the pressure reading while that happens. So useless on the track, UNLESS you look at the monitor WHILE you're driving.
But the biggest reason why I like the thing is in daily driving:
I am not the best guy in checking my tire pressure regularly but on the other hand my tires seem to be "nail magnets". So far twice I got a warning from the monitor where I had caught a mail and was SLOWLY loosing pressure.
In both cases I safely drove to my tire guy who patched the tire before anything bad could happen.
So big two thumbs up for day to day safety.