I’ve not used it personally, but I have been with colleagues who’ve used it in New York. I think it’s brilliant, traffic was heavy, it was cold and raining, yet we found a cab (limo) in two minutes and could watch its progress as it inched towards us in the Manhattan traffic.
It’s what London needs. It’s SO frustrating trying to find a cab when it rains or around shift change time when you see plenty of empty cabs with no “For Hire” light on and no passengers sitting inside. In this respect NY is ten times worse and I find the yellow cabs to be cramped and often rather smelly, especially if the driver is enjoying his kebab while driving you!
Personally I think Uber and London cabs would make a good fit, so instead of trying to beat them, join them, or, if they’re so keen to pay tax (and the fare is higher when using Uber) develop a competing system exclusive to them!