In a movie theater, it's probably a good idea. I know I could have used such a jammer when I was house manager at my college's theater all those times, and one of my friends in the cast was an epileptic, and could have a seizure from the cameras that people use even after I tell them not to use them.
Personally, however, I wouldn't want one in cars. I've been rear ended by a guy driving on his cell phone myself, and I personally don't like the idea of having a jammer in a car.
Personally, I'd much rather have stronger penalties for people who talk on their cell phones while driving, unless it's an emergency.
Instead of having fines of only a couple hundred, make it a thousand, or a couple thousand. Have it go against your license like unpaid tickets and car accidents. Enough times, your license is suspended, and who knows, you're prevented from having a cell phone for a couple years.
But if I'm riding shotgun or in the backseat, I'd like to know I can use my cellphone if I need to or want to. And for me, I only use my cell when I need to. That's why I have Virgin Mobile. It's prepaid.