To put a quick perspective on things, I have ordered about 1.2L worth of goods from Flipkart. Sampled practically every category they have.
And I'm going to be a bit judicious in saying that their service is good when the sourcing is under their control. For large ticket items, it's better to go to your local dealer.
Problem 1: Payment gateway is extremely fickle and times out. The effect is that your account gets debited, but money doesn't show up with Flipkart and a reversal takes 10 days. If you have a large order, you might want to forget it. I have had to roll back three of my orders and try two again - didn't bother re-ordering the third time, it was too steep a climb of faith for me.
Problem 2: 'In stock' may or may not be true. I placed an order of 46k for an AC and a fridge for my folks, I thought the service was going to be really good but FK simply could not procure the order - at all, not in time. And somethings just take so long that you give up and cancel the order in frustration. Happened three times to me (including this once).
Problem 3: Flaky pricing - at some point the prices on the website is much lower than retail, and sometimes much higher. Now it's mostly much higher.
Problem 4: non-existent support for IT products - I have a Gigabyte motherboard that took six weeks to arrive. The first one arrived in six days, it had a curving problem (see my build log) and FK took it back, and then the replacement took four weeks to arrive. You better know people at manufacturers', or just buy locally. Save yourself the hassle. Plus most large-ticket items will probably be cheaper at local, or specific IT vendors such as Prime.
For everything under about a grand, FK is pretty good, for anything above I would wonder if they've got a sorted back end.
In the inital days after launch they had a small, but reliable selection. I think the pressure of widening the portfolio without taking on inventory cost has taken a lot of the control out of their hands. I imagine they also squeeze manufacturers quite a bit (nikolian on these forums has mentioned this) and therefore are not the first choice to fulfil demands when a crunch situation happens. I got the same AC locally in Calcutta at the same price as FK, though I had to manage a lot remotely - the main reason for choosing FK is the service (which I experienced when I bought a TV off the site) but they couldn't work out a way to deliver the product.