I have an Ofna Fail Safe, and it works well when your car goes out of range, and then also when there is a low battery warning. I would say that it is worth the money. The only thing is that is you use rechargeable batteries, the you have to have an rx/tx pack (5 AA rechargable battery pack). If you only use 4 batteries, then the Fail Safe will always flash low battery. I am currently using only 4 batteries, becuase not enough money to buy a rx pack (aboutf $25). I know that I got a bit sidetracked, but I hop that it helps.
-Pat (redhatman)