My MX 500 is still going relatively strong, except for some left-click issues.
That's why at home I replaced it with a G400, and am using it at work instead.
Might have to open her up and see if there's any gunk blocking the action, or if the clicker is worn out.
If it's a clicker, I guess it's just a matter of soldering on a new one in place of the old one..