The only time I've ever really used them was with a group, late at night when we needed to catch a "last train" or something and nothing else was available.

I did pick one up from Heathrow once I'm ashamed to say - I asked for Chelsea and he had no idea where it was - but he quoted me a fare! He didn't even know if it was towards London or not and I ended up having to direct him because we were off airport property before I realised I was in the Twilight Zone.

S.