There's a lot of potential gain from car-to-car communication, but I also see a huge risk. I can be reasonably confident in my car's own sensors, but only God knows what my car is being told by other cars about the world; the opportunities for malicious activity there are huge.

It's hard to imagine a system that uses V2V that doesn't rely on some manner of trust, like a closed/encrypted system with car manufacturers having CA-like signing authority or something. And if I've ever learning anything, it's that car companies are not great at network security.