I'm amazed Apple doens't provide this feature, thankfully there's Mail Unread Menu.
As most experienced mac users have their dock hidden, the most obvious way to see if new mail has arrived is by unhiding the dock (by mousing or shortcut) to see if there's any red badges on the Mail icon. This can be fairly disruptive for people like me compared to just a quick glance at the menubar.
This plugin also has an edge over the dock icon by allowing you to display a badge only for the mailboxes you need to keep an eye on. That's a feature Apple would be unlikely to implement, so i hope they never create a menubar icon for mail at all, as the 3'd party product they end up killing is often superior in terms of functionality (eg. Desktop Manager).
Lots of apps put some stuff in the menubar, but very few is as useful as this one, and the price is unbeatable (but donate if you want to show your appreciation).
I've been using this plugin for years, and it hasn't given me any problems so far.
A nobrainer install in my humble opinion, and a solid 5 across the board.