Terrible
I tried IndiGo for the first time this year after hearing that it was decent from other people. That was not the case for me like many others clearly. I was trying to do their online check-in so that I would save my family some trouble later on, and after painstakingly typing in my details multiple times for almost an hour: I went through all of their add-ons (twice for some reason), seat reservations, etc and at the very end, they decided to tell me that we were not eligible for online check-in. Then, after all that, we tried to talk to their customer support and they weren't helpful at all. We had some separate issues with our flights before this, and their customer service was the exact same then as well. They wouldn't answer our questions properly, they took their sweet time and rushed us after 1 minute, they started talking about completely irrelevant issues multiple times with multiple different people. All in all, I would never recommend this to future fliers.