Research Hypotheses

What facets of XP worked well and were easily accepted by the customer (including some practices that we expected that the customer wouldn't like)? What practices proved their worth when requirements or conditions changed? What practices were difficult to implement? What modifications did we make to some of the XP practices and why?

Obviously, we don't have definitive answers to these questions that can be extrapolated across all projects, but our account adds one more data point to the sample.