We are doing an adult only in November but we've done a cruise (Dream) the last week of August and are doing another (but 7-day) the last week of August going into the Labour Day weekend next year. Our first cruise was nowhere near full due to Hurricane Irene passing through just prior but it is a slower or week with less demand (as seen in significant drop in price for that week).
For us, it is a great way to finish the summer and enjoy some significant savings.