I have used NTL for about 30 times now. This is a really a wonderful service.
If you want to travel in night, women traveling alone, etc then NTL is a reliable one. To be honest in last one year I have used all call cabs of Chennai. There can be only 3 names you can rely on Fast track, Bharathi and NTL. I would rate all the 3 equally in terms of ON time arrival at your place, urgent need of cab, and cost.
One unique disadvantage with NTL is their cost . Agreed that all the 3 operators are priced same but it is NTL who increases the price 1st and later its followed by Fast track and Bharathi.
I have never got NTL cab late. They arrive 30 mins earlier most of the time. Also most of the drivers are polite enough. I have cancelled the cab once and remember you need to cancel it before driver arrives your place(say 30 mins before) else the driver will bargain with you.
Also, they have panic BUTTON and remember to press it when you are in danger(I never have tried it;).
Assume a case where you booked indica and got Tavera then you can speak with driver to get it at cost of Indica since it is you who clearly booked Indica only. If the driver does not agree then please call them at the start of journey.
I have had both experience of getting a Tavera at indica price and also driver fighting with me for not giving me a discount. So that particular driver was rude. But in general NTL is good.