We are using them for the first time on our next trip, but they came highly recommended. We have already registered on their site (you should do this early, as they sometimes have too many deliveries to fit more orders in on a particular date)
When you register, it has a space for your hotel name as well as comments, such as " arriving late, please leave order at bell services." The Disney staff will keep your order there until you arrive - they have a fridge for cold items. We are actually using the service because we will be arriving after all restaurants are closed.
You can save the cost of the delivery charge just by ordering a case of water!