Who knows better? You or your customers?The answer is that it depends...
In a corporate role, or one in a large organization, market research consists of occasional focus groups or surveys, if at all. More often the decisions are made by executives who "think" they know the customer and can spot the opportunity. This is a crapshoot - sometimes the hunches are right, and sometimes they are painfully wrong. The worst part is that when the decisions are wrong, they can be a costly waste of time, money and company resources.
In a startup, many founders fall into a similar trap. They assume they know the market, or use their own opinions. These are most likely wrong! The successful startups all are grounded in insights about the market, and use that wisdom to guide the next steps. Some effective and inexpensive ways to validate ideas include interviews, and leveraging tools such as Amazon's mTurk.
Customer insights should bookend all product development and marketing decisions - at the beginning and and the end. Even if customers never expressly say they want "product ABC", as a leader, you have to read the hidden, non-verbal cues to uncover the truth.
What market research did you use in your startup? Share the details in the comments below.