I recently started using it when I had to revamp Let's Sermo's framework etc. It was quite easy to use, but I did not customize it.
Download: WordPress › Maintenance Mode « WordPress Plugins
If you want to make changes to your blog, or do some extensive remodeling, you probably want your visitors to turn a blind eye. With maintenance mode, you can inform your visitors of your renovation, and when you'll be back online.
There's also the option to use your theme's 503.php file to replace the maintenance message. That means you can take your blog offline WITHOUT sacrificing branding. In addition, you can add a lot of cool code.
You can also exclude certain pages from maintenance mode, say a subscribe or about page, which allows visitors to access critical parts of your website.
The great thing about plugins is that they often come with options for crazy customization. So do you use maintenance mode?