8. We`ve learned that we should only ever use credit cards that have barely a hundred dollars or so of credit available to secure the room, since places like the Serrano will charge undiscounted rates -- and add the difference -- to guests who prepay and book through discount brokers such as Tavelocity, Hotwire etc.One final warning: The Opal stocks its rooms with a booklet of local businesses restaurants etc, which is probably very useful.