First you should understand what is a user cache to decide whether you should enable it or not. User cache means you cache data(save data) into APC. Now this could be an object, variable anything.
Lets say you get user profile from database, and you want to cache it so the next requests wont be needing to fetch from db. You would cache it in APC after the first fetch from db. this is an example of where you might use APC user cache
$userObject->name = "Mike";
$userObject-> age = 12;
apc_store("user", $userObject);
You can retrieve the value, with the key which is user.
var_dump(apc_fetch("user"));
THats user cache
Have a look at this site about Perfect APC configuration: