Sounds like someone is using too much hand cream. :roll:
Need to be careful what you use to clean those button surfaces. Alcohol can damage the screened surface. Never use anything petroleum based like goof off or lighter fluid. Good old Dawn and warm water usually most body oils off contact surfaces. is the steering wheel itself sticky?
+1 with the Dawn detergent. I have an older MB (2000) that developed sticky buttons. It turned out that there was a coating on the buttons that had degraded. I ended up removing the coating to eliminate the tacky feel. Doubt that would apply to you if you have a MY15.
Just bought a 2015 Supercharged last month. Seemed like a lot of the buttons had some of this sticky feel. Was told that cleaning them would not help. The really bad ones were replaced under warranty prior to me accepting delivery. There are still some with mild stickiness that will eventually need replacement.