Will years of writing on it ruin its surface and/or lessen its performance?
If so, what material should I use?
I am a happy owner of Wacom Intuos 3 bought more than 3 years ago.
The grey surface on it is pretty durable, made of sturdy plastic with several layers, feels laminated. Actually, recently it just came off the base by itself, there is hardly any glue holding the grey writing surface to the poly?carbonate base.
After several years of medium use, there are hardly any scratches. I clean the pen tip before each use with fingers. When working with hands, the sweat stays on the writing surface, which mixed with dust causes similar dirt deposits as on a regular mouse.
The final verdict:
Overall: a well designed device not needing any surface protector, which would only deteriorate the "smooth paper feel" of the original grey writing surface. Compare writing with a plastic stick on a smooth paper on a table (Wacom) against glass (protector).