This little gadget looks pretty cool and could possible save a lot of lives. It kind of looks like it is a GPS device as well. Now that would be a nice feature. I think if you could download apps it would be a must have for cyclist.
For $12 you can buy a mirror for your helmet and get about the same thing, if not better. I live by my mirror. For $300 it looks nice, but a lot of $, and you still don't have a computer, so would have to buy the 1000 if you want a computer too. I have a Edge 500 and a mirror and that works very nicely.
It's definitely a useful device, although it's not necessary for obvious reasons, especially at that price point. I'd buy one if I had the money to spare, but I wouldn't hesitate to keep putting off such a purchase otherwise.