I've had a good experience with this plugin. My client needed a way to demo a bunch of different colors of websites to his potential clients. This plugin allowed me to very easily offer that capability.
One small bug - on the frontend, try switching to any of the stylesheets not in the first 5 added and it will revert to the default stylesheet. A quick fix in the plugin code (remove || ($stylesheet_choice > 4) from line 37 of wp_user_stylesheet_switcher.php).