I might use one of these at some point. I wouldn't use it to leave the car on a hill while I went into a resturant to eat dinner but it might be useful to hold the car while you tie it down on a trailer for example.
Park-Lok Hydraulic Brake
That seems to be a popular device among the rock climbing crews, but I didn't buy it because it's too bulky (imho)
Besides, you can't really compare a $20 parking brake found at places like JC Whitney to a $180 industrial strength ball valve.