What a great idea - brilliant!
I saw one at the last gun show. I thought that the strap that went around your arm was a great idea. The only problem I see is if you have to go to a different weapon, your going to have to shed that thing in a hurry. That could be a problem in a hot situation.
Theoretically, they'd come in handy if, for example, your car was surrounded by violent bikers who attacked you and your family while riding in your SUV, provided you weren't already trapped amongst a stateful of gun-fearing wussies in NYFC.