So today I've personally handed over some 13 letters at the CeBIT trade fair in Hannover.
My experience varies from case to case. A number of the respective recipients simply received the letter and told me they would forward it to the respective department.
The best experience so far was X-Micro, where I met the Vice President and had some discussion with him about what this all was about. Apparently he was quite happy to hear that it is not about license fees and neither about patent infringement ;) Anyway, we'll have to see what kind of practical results we will see in the upcoming weeks.