Known Bugs
 
1.   Gallery2 doesn't work with PHP5+APC.  There is something wrong
     with the way methods are restored in some edge case I haven't
     been able to figure out yet.
     To reproduce install gallery2 and click down to an individual photo.

2.   apc_store() probably needs some checks to skip trying to store
     internal classes.  Something along the lines of:

     if(Z_TYPE_P(val) == IS_OBJECT) {
        zend_class_entry *ce = Z_OBJCE_P(val);
        if(ce->type == ZEND_INTERNAL_CLASS) {
            php_error_docref(NULL TSRMLS_CC, E_WARNING, "Cannot cache internal objects");
            RETURN_FALSE;
        }
     }

     in the apc_store() function in php_apc.c but I am wondering if it needs to do more
     than that.

Enhancements

1.   Some faster platform-specific locking mechanisms wouldd be nice.  futex support
     for the 2.6 Linux kernels, and/or x86-specific spinlock support.

2.   The optimizer needs a lot of work.  

3.   Assert() elimination in the optimizer when some debug flag somewhere isn't set.