One of my office colleagues bought a new Tata Indigo and has been using an Indigo Petrol (GLS) for more than 2 years now and has had no issues till now. In Delhi city, it gives him 10 to 11 kmpl with A/C on. On the highway, he gets 17 kmpl with A/C. If you are looking for a smooth pickup and a comfortable drive, go for the petrol version. I think one easy formula to decide whether you want to buy diesel or petrol is to think of the average running of your car per day. The showroom salesperson told him that if your daily running is less than 25 to 30 kilometers, you should not go for the diesel variant plus Tata Motors has bettered their petrol engine after getting the 32 bit MPFI engine. It's a much smoother drive.
Two areas of improvement:
1. Gear is not as smooth as a Maruti car.
2. The plastic parts in the car (door handle, arm rest, petrol/boot opening levers etc) can be much better
But it's the best value for money and incredibly spacious and therefore a positive review for Indigo Petrol from my side but I am not sure about Indigo Diesel as there are a lot of complaints about it.