I have the Garmin RVR315 (no light, somewhat affordable).

1. Nominal battery life is something like 8hrs. Turn off during lunch, perhaps recharge as well.
2. During daytime, I don't see the point of a rear light. At dusk or night, I use a Petzl Bindi (14 days battery life)
3. I really like the idea of a radar. In my case is more useful to detect road cyclist about to overtake me on a path. Is also useful to detect cars but it doesn't replace rearview mirrors - if a car slows down and matches your speed (frequent when car can't safely/legally overtake), its signal often disappears. I believe this no longer applies to the most recent models which show "trailing" objects.

4. Seems to be like a silly gadget until you get one, but once you do, there's no going back. It may not be *actually* safer, but somehow is very satisfying.