Red_Eye_Racer: You know, you should've made this a poll where you can select multiple answers. I'm really torn between "never use it" and "like to eat poop."
I'm with neon_duke on this one, though. I would never use the handbrake when trying to get a good time, but it's fun to fly across the finish line backwards -- especially when you're just a little bit ahead. It's funny to win by 0.2 sec when you're going backwards.
Also, for crazy people trying to get 100% with the least milage, your car doesn't accumulate any miles when going backwards. If you're ahead in an endurance race, you can save several miles by getting up to 200 MPH on a straight then using the handbrake to do a 180. You will maintain a lot of speed going in reverse if you do it right, but you won't be accumulating any miles. You can just drive backwards normally, but then you only have about a 50 MPH top speed.