Not quite big enough to stay in charger, can come out a bit and loose contact if driving along road that is less than smooth. Ok if battery runs flat and I need to make an urgent call, where I'd need to pull over and park up. being stationery it will stay in and get power, as long as don't pull on cord too much.
Could do with a thicker end so its a tighter fit in the cigarette lighter, so not loose enough for the contact on the end to move and lose power. At least there is a red light that comes on, so you can tell if it has moved.