I think it's still a good extra eye in the city. I will note some special circumstances that you have to watch out for.
1) If a car behind you matches your speed for whatever reason (waiting to pass, about to make a right turn and doesn't want hook you, etc.) it will disappear from your display but it's still there.
2) if you pass a drive way or street and a car pulls into the traffic lane the Garmin might not pick it up in time as the car accelerates and passes you.
3) In roundabouts the angle might not pick up a car closing distance so be aware of that.
4) Some commercial burglar alarms (?) seem to set off my Garmin and give a false alarm. But if you cycle the same route over and over you'll learn these false alarm spots.