Based on bench and animal research, patients with ADPKD were generally told to avoid or minimize coffee. The reasoning comes from data that shows a relationship between arginine vasopressin (ADH) and cyst progression. This relationship has gained validity with the repeated successes of tolvaptan in ADPKD. Arginine vasopressin binds V2 receptors and then transmits intracellular signals via the second messenger cAMP. Caffeine slows the degradation of cAMP, amplifying the signal. See this link for a nice review.
Today, word comes of a 150 person, four-year study that was unable to find a link with disease progression and coffee consumption. Yea!