I have been using Vodafone for around 6-7 years. I have seen it undergo shift from 2G to 3G and now 4G. I chose this provider over others because I had a bitter experience with Airtel and did not want to choose it again, and the only alternative was Vodafone.
In terms of reliability it fared well according to me. The signal strength is mostly good, but yes sometimes people have complained me of my not being reachable, while I had a sufficiently good signal strength. So there is room for improvement.
Overall I had good experience with them.
P.S. I am giving them 3 stars as there is ample room for improvement in areas such as introductions of services. It does not introduce new services as fast as Airtel does.