I have: many, many times. Generally, the triangulation fails (seeing as the cells overlap so much in Central London). So you give your details to an operator. And then, nothing. No cab in a suitable distance at all.

Seems the uptake with the drivers themselves is pretty low (surprising, seeing as they can only really make money from it...) So the only times and places it's useful are... When and where there's a shedload of taxis already kicking about. Give it time, it will work eventually. Just not yet.