At what point in the buying process is surveying the buyer/ customer most effective?

Unfortunately there is not a "best time" to do a survey. And the more focused one can be in what decisions one is making from the research, the better one can pick the right moment and method of collection.

First -- I always recommend separating feedback collection from marketing & sales efforts. The objectives of the two exercises are usually at odds with each other. That is not to say that sometimes we get feedback during a marketing or sales effort, or that asking for feedback we won't sometimes gain new prospects. Just that in most cases trying to sell or market while gathering feedback will bias the information collected.

When to survey customers depends very much upon what feedback one is seeking. For all market research projects, especially surveys, deciding upon what business question you are answering and what decisions you will make upon the results must be clarified first. Then one can determine the best point in the sales cycle to gather feedback and how to initiate the request. For example, to gain feedback on how well product messages resonate with prospects, early in the purchase cycle might be most accurate. Once the customer has more experience with the company's products and customer service, the message interpretation will be biased by that experience. But if one wants competitive feedback, then catching that customer after exposure to multiple company's offerings will be necessary.

Some of the best surveys are very specific and triggered by a customer action. For example, a customer who spends a few minutes on a feature comparison web page but doesn't proceed to selecting one of the options could be interrupted with a single question survey about what they were seeking right at that point in the purchasing cycle. Cart abandoner surveys are similar -- if someone doesn't come back to their cart within 24 hours, an email offering a reminder, assistance, or feedback can be quite timely. Whether one is truly asking for feedback, or is really trying to move along the purchase will guide the engagement with that customer.

Similarly, most product feature feedback requires some level of experience using the product. Setup feedback is best done during or shortly after setup. Feedback on the reporting an analytics capabilities is best collected after the customer has had time to collect enough data to generate an interesting report. Smart product managers and product designers will enable their products to help feedback collection by tracking those trigger events.

Gaining customer insight evolves over time as the maturity of the customers, the products, the customers and your own team's knowledge grows. It is a never ending and always revealing experience.