There are some often mentioned benefits associated with user-centered or user integration methods and approaches (e.g. participatory design, co-design, empathic design), such as better product usability, lower risk of market failure, better company image or a larger market share. To name just a few. These effects are sometimes not measurable at all, not quantitatively or qualitatively, and in some cases they are not even recorded.
My questions regarding this topic are:
1. What effects/ benefits or impact of user-centered design and user integration methods do you know? Maybe from your own experience.
2. Are there publications where effects have been measured quantitatively or qualitatively?
3. What are the downsides? Probably higher cost; but is this always the case or does it depend on other conditions?
What is your experience with performing these kind of methods?