I had been using lot of call cab services in Chennai and can trust Friends Track. The biggest disadvantage is the drivers attitude and availability of cab. More often than not I have got a response NO Indica available esp in night times. Hence you may have to rely on big cars but again the problem is lack of good cabs.
If you ever want to book Friends Track do it but well in advance. Do not go for them if you want a cab within 1 or 2 hours.
Advantage with them is that because of the competition, they are the cheapest after Taxiforsure cabs in Chennai. They are reliable if booked well in advance.