In my case for my latest project, BotMeNot , it's crucial. I'm still in the beta testing phase and I can't keep properly developing without real people coming to test my product and giving me honest feedback.
It's hard to find a replacement for genuine feedback because customers don't approach the product the same way as developers do. I think it's impossible (or extremely difficult) to predict all use cases and customer behaviour before actually real people give you feedback.
It is obviously important, but the most innovative products offer something the user didn’t even think they needed. I think the real behaviour of customers gives more information about their needs than concrete feedbacks from them.