Using multiple memcached instances with options in combination with persistent connections might be confusing:
<?php
$a = new Memcached('memcached_pool');
$a->setOption(Memcached::OPT_COMPRESSION, false);
$b = new Memcached('memcached_pool');
$b->setOption(Memcached::OPT_COMPRESSION, true);
$a->add('key', 'some data');
?>
You might think that connection $a will store everything uncompressed, but this is not the case.
The persistent connection options are changed by the second object creation.