Yea, Flipkart's return/replacement service is quite awesome. Last time I placed a return request (at around 10 AM), the pick up guy came within next two hours. I wasn't expecting them to come so quickly. Anyways, I had carefully packed the earphones by then, handed it over to him. He started opening it, I was like "Dude! what the heck, why are you opening the package, now it will be your responsibility if anything goes wrong/missing".
This was the first time I had returned something, I was under the impression that it will be returned to some Flipkart warehouse, they will check it & issue the refund. But he did some minor checking there itself, took the package with him & I guess later when he returned to the facility in the evening, updated the collection in the system & Flipkart issued refund right after that.
As far as replacement goes, I guess they send the replacement first & then pick the defective item at the time of delivery. Having their own logistics service sure smooths this whole process for both parties.
Although I have Flipkart First subscription, but there's hardly anything in their Delhi/Gurgaon warehouse, most of my past orders have shipped from Banglaore/Mumbai. So in a day delivery is almost never available to me.