I own and use an arse trap.
It's quite handy, especially when using small primes and I want to be able to tuck the camera/lens under a large coat but still have quick access to it. I have a large jacket that will accommodate my D300 with a lens fitted inside it with me (though it does look like I'm pleased to see someone) and I can get a few other primes in my pockets. Quite handy when doing street stuff.
I only got it through coincidence. I'm not sure I'd go out and buy it myself.