Exchange 2K users sharing a mailbox

A client requested that I setup about 15 users with different login names to share one mailbox in Exchange 2k. I setup this up by creating a mailbox and gave permissions to the group of users under the advanced tab in Exchange. It seems to allow more than one user to be able to attach to the mailbox and use it at the same time. Is this going to be ok ? or is it going to be my worst nightmare?

Any comments or suggestions ?