I have been using airtel 4G since 3 to 4 years l.
Till now I am never upset with airtel. Airtel provides free internet to most of all users ( approximately all users) and customer support is also good.
The recharge plans are also in average range.
But in many places you cannot find airtel networks though I am in Mumbai now but when I was in Uttarakhand ( UP West ) I was facing problem of airtel networks there were no 4G network but only 3G network was available.
Airtel may have good speed and quality but till now it is not a true 4G network.