Wow, that's just silly then. The user caching part of APC is practically the sole reason I use it; it's by far the fastest (non-distributed) caching mechanism available for PHP right now, several orders of magnitude faster than memcache or redis since the round-trip time is zero.
Is that what you've found running memcache/redis on the same server? I've never actually used APC for user-caching.
Yes, YMMV depending on your environment, but I did test this carefully before making the decision to use APC for data caching wherever possible. IIRC I saw up to thousands of fetch/store operations in just 1ms using APC; but I'd suggest you to benchmark it on your own and judge by yourself.
Are you sure about that? I'm getting better performance with XCache, and it has everything that APC has plus some other useful features like deleting keys by prefix.