I have kept an ACR signal mirror on my boat for at least 5 years, and it looks as clear as the day it came out of the package. It is kept in a cubby hole at the helm,just siting, no case. CountyComm offers a nice one that comes with a nice case.
The CountyComm one doesn't have a retroreflective aiming device - I checked with them. Looks like a nice mirror though.