I understand that people at bitcointalk are aware of this maintenance, but what about the people accessing Sinbad directly and not using the bitcointalk forum, they will just think that that site is down. It will have a serious bad impact on their business.
I completely agree that placing a sign that the service is currently under maintenance and that it will be reactivated after several hours would be appropriate with providing access for users who have pending transactions related to how their transactions are executed at that time.
I think the current behavior is understandable if they are making major changes such as renaming the service and stopping the current service.