I get errors with apc result and query cache enabled:

Warning: unserialize() expects parameter 1 to be string, array given in C:phpincludesDoctrineQueryAbstract.php on line 1097

This modification in Doctrine_Cache_Apc::fetch() solved the problem:

    public function fetch($id, $testCacheValidity = true)
    {
        $tmp = apc_fetch($id);
        if(is_array($tmp))
        {
            return $tmp[0];
        }
        return false;
    }