How can I get Users by using Get-MailboxPermission?

I want name of users who have access rights of a mailbox. I wrote like below.

$users = get-mailboxPermission -Identity $Identity | format-table User

and checked on console typing "$users" and it returned several user names correctly.

User

-------

xxxxx\Adminstrator

xxxxx\Matt

But when I use these values as String, they turn out enigmatic string"Microsoft.PowerShell.~"

How can I get these values as String?