Just thought this info may help some of you out there.
One of the remote keys on my W211 E class was starting to get a bit tatty, with the plastic buttons starting to disintegrate, so I took a risk and purchased a used key from Ebay.
Now I know that they supposedly cannot be re-programmed, as some sellers will have you believe, but you can swap the circuit board from your old key to the new housing.
This has now enabled me to have a key that looks brand new for only Â£10 !
You do have to be very careful when removing the old board, but its not difficult and I did mine in 5 mins.
Hope this info helps.