I have now been buying books from Flipkart at least twice every month since the last one and a half years and I can vouch for their service quality. I think their service is top notch and I don’t think any other portal comes anywhere close to flipkart. (Infibeam is a distant second) I have ordered more than 40 books worth over Rs. 15000 from them in this period and every time, their packaging, delivery timelines, etc. have been excellent. Believe me, in such a long time, I have faced only 1 issue when they were unable to send me a book by Christopher Hitchens because it went out of stock and they kept on trying to get an imported edition instead. But they were very polite to cancel my order and refund the payment promptly so I had no problems. And this did not change my trust in them at all and even today, I prefer to order from Flipkart even if the book is priced Rs. 20-30 higher than competitors because I know that it is the better and reliable service that I am paying for.
And since some time, Flipkart have started their own delivery service and so they also do not rely on 3rd party courier services for delivery and I can promise you that I get most of my orders before time now. Only downside is, I find "some" books are priced higher than competitors like Infibeam and in cases where the difference is large, I do order from Infibeam, but my first priority is Flipkart always.
On the whole, I can say it is a boon that we have such a service in India and I hope it becomes as big as Amazon very soon.