Haven't found a true successor yet, so it has to last a while longer.
Logitech still makes MX518 so you can just buy a new one. They have changed the base of the MX518 to make it better too. I have an old version and the newer version. The base now has large skate pads instead of small feet and the text underneath is now silk screened into the base instead of on the sticky tape stuff so has no chance to come loose. A couple of the feet fell out of the old version and is how i ended up with a free new version.