On first impression this plugin works and gives a nice splash message with a full screen darkened image behind it.
However, users can still log in or browse the site if they came in with an innocent bookmark like mysite.com/wp-login.php . Most post or page links are blocked, but once someone is logged in (or their cookie says they are) they would never know they weren't "supposed" to be there and if they stumbled across a page that was in development, they might consider the site broken.
So I guess it really serves as a polite notice and not a way to actually prevent anyone from seeing the site when it is in maintenance mode. Am I missing something or is this really true for others?