They were sending out free credit emails every weekend for £45 (£15 off three separate trips); no minimum spend, no qualification criteria, just free trips. That's well over £150 of free credit some months!
Even with the free credit though, I still used Uber because Karhoo was a terrible experience:
* The time estimates were never correct, no matter which car provider you selected. You were always waiting longer than 15 minutes for a car. * Dial-A-Cab always said there was a car 7 minutes away but that was a lie. You then joined a queue to be assigned a taxi, and that car could be any distance away (I never received a Dial-A-Cab taxi in under 25 minutes). * There was no continuity in the rules between providers. For example, cancellation fees were apparently on a provider-by-provider basis. * The prices were often far in excess of Uber, but for the same cars that UberX uses.