I agree, some cushioning is vitally important. As I prefer to be barefooted and my floor has no carpet or padding whatsoever my heels started to hurt at the end of the very first day. On the next day I felt that pain from the very beginning, so being productive was out of the question. Contrary to the general consensus I got myself a really cheap gymnastics mat for less than 20EUR which is only 2cm thick, instead of an expensive special purpose mat. Within 2 days the pain was gone and standing/working for more than 10hrs a day was not a problem anymore. So get a mat or wear shoes if you plan on trying a standing desk configuration.