Most of my driving has been done in fairly heavy vehicles with normal profile tyres - as in a 75 profile. When a tyre starts to go flat, it's pretty obvious with a heavy car and a high profile bit of rubber. The sidewall bulge sticks out like dog's balls, braking performance suffers, steering can be heavy and you can even feel a wobble through the wheel or the seat.
I got the shock of my life yesterday when I took a little car to the petrol station for a top up. There was an air filling station in front of my spot, so after filling up, I drove over and checked the tyres.
They were 1/3 to 1/2 around where they should have been. I couldn't believe it - the tyres showed no visual sign of being underinflated, and they're only a few months old. When driving around, the handling of the car gave no indication of flattish tyres - I'm not sure why that should be the case. Light car? Good suspension? Well built tyres? I know instantly when the pressure is getting low on the other car - it provides good feedback - plus walking around the car and eyeballing the tyres is a dead giveaway.
As soon as I got home, I ordered some of these. They cost me less than five bucks delivered. I don't know how good they'll be, but I'm going to give them a crack.