The source of the most useful feedback is a potential customer with a reputation for rudely telling people "how it is", and a ton of knowledge about the domain, and no reason to be nice to you. The worst source of feedback is your friends and family, especially if they aren't knowledgeable potential customers.

Good feedback: "Why would I pay for your product? I wrote a bash script that does everything you do already. Do you have [feature x]? I'd pay [$y] for that."

Bad feedback: "This is so cool! I could see myself using this if I needed to."