I am using freecharge website since 3 years. Its good and fast. I really like the section of View Plans which lets you select the plan which suits you.
Moreover, in case of failure they store the amount in your account and you can use it for later recharges. It actually shows the option debiting the amount from your freecharge account.Plus they also have coupon codes through which you can waive off your certain amounts on recharge.
The only thing which sometimes bother is that you have to drop email in case of any issues.Otherwise its a good portal for recharges.I have used other portal as well but the experience was not that good.
The features are user friendly and good and very very comprehensive. Its one of the first website to offer such service and I have hardly faced any technical glitches. Kudos!