I have used Ola Cabs several times and have found the cab service really good.
The cabs usually arrive at the given time, without any delay.
I had to face delay only once, but they compensated it by offering a free upgrade to a sedan cab.The billing system is also transparent.The drivers are always polite and helpful.
I mainly use Ola for cab booking, although I have used TaxiForSure and Meru a couple of times.The mobile app is also simple and easy to navigate.The best part about Ola is that it provides vouchers for referring users.A really good service.:)