No, doesn't work where you cant get a signal. But that depends more on your carrier. I suspect it costs data usage but I have unlimited data so I don't know how much.
It also doesnt' work well if there aren't any other users around - the benefit is it provides realtime up to the minute info about road congestion, police, road hazards so if there aren't any users nearby there aren't any reports. Then it just becomes a regular navigation app.
I used it to report a state trooper working the road in the middle of South Dakota and five minutes later I got a "thank you" message from another driver.