I am running multisite with 3.5.1. I was unable to send messages with your plugin. I kept getting this message '0 messages sent 1 error'.
I enabled debugging logging and saw that I kept getting table not found errors:
"WordPress database error Table 'XXXX.wp_XXXXX_pm' doesn't exist for query ..."
In all the plugin php files I changed all instances of
$wpdb->base_prefix and it works great!!
I do not know if this was a proper fix or if others are having this issue or if this is due to multisite, but I just wanted to let you know and I wanted to find out if this was an appropriate fix.