I am using Reliance GSM for the past 4 years. Overall my experience is not bad with its network. Day by day they are improving their network. Particularly their 3G service is quite good in Kolkata.I get H+ speed in most of the areas here at Kolkata.
Also I am using Reliance SIM in my dongle which also gives superb speed. They have varieties of voice and data plans which suits my requirement. Their Rs.499 5.5 GB 3G plan is the best in the market which I use on my dongle and Rs.198 1.1 GB 3G with Free FB+ WA+ TW. The only problem with them is they frequently deduct voice balance from main account though after registering complaint they soon refund it.