Fix for version 1.0.1.

gistfile1.php

<?php
/**
 * Plugin Name: User Switching | Admin Bar
 * Plugin URI: http://markwilkinson.me
 * Description: Build upon the User Switching plugin (http://wordpress.org/extend/plugins/user-switching/) by John Blackbourn and adds a dropdown list of users in the WordPress admin bar with a link to switch to that user.
 * Author: Mark Wilkinson
 * Author URI: http://markwilkinson.me
 * Version: 1.0
 */

/* alter the wordpress admin bar */
function pxjn_user_switching_adminbar() {

	/* include plugin file to make this work on the front end */
	include_once( ABSPATH . 'wp-admin/includes/plugin.php' );

	/* check whether the user switching plugin is active */
	if ( is_plugin_active( 'user-switching/user-switching.php' ) ) {

		global $user_switching;

		/* load the global admin bar variable */
		global $wp_admin_bar;

		/* check whether the current user is super admin */
		if ( is_super_admin() ) {

			/* add admin bar menu for switching to a user */
			$wp_admin_bar->add_menu( array(
				'id'    => 'pxjn_switch_to_user',
				'title' => 'Switch to User',
				'href'  => '#',
			) );

			/* set some arguments for our user query */
			$pxjn_user_query_args = array(
				'role'    => '',
				'orderby' => 'display_name'
			);

			/* begin build of switch user url */
			$pxjn_switch_user_url_base = wp_login_url();

			/* create a new user query */
			$pxjn_user_query = new WP_User_Query( $pxjn_user_query_args );

			/* store results from user query */
			$pxjn_users = $pxjn_user_query->get_results();

			/* check we have users */
			if ( ! empty( $pxjn_users ) ) {
				
				/* loop through each user */
				foreach ( $pxjn_users as $pxjn_user ) {

					/* get all of this users data */
					$pxjn_user_info = get_userdata( $pxjn_user->ID );

					/* build query args for this user */
					$pxjn_switch_user_query_args = array(
						'action'  => 'switch_to_user',
						'user_id' => $pxjn_user->ID
					);

					/* build menu url */
					$pxjn_full_menu_url = $user_switching->switch_to_url( $pxjn_user );

					/* build menu id for each user */
					$pxjn_menu_id = $pxjn_user_info->ID;

					/* add admin bar menu to create a new appraisal */
					$wp_admin_bar->add_menu( array(
						'id'     => $pxjn_menu_id,
						'parent' => 'pxjn_switch_to_user',
						'title'  => $pxjn_user_info->display_name,
						'href'   => $pxjn_full_menu_url,
					) );

				} // 

			} // check we have users

		} // check we are super admin

	} // check user switching plugin is active

}

add_action( 'wp_before_admin_bar_render', 'pxjn_user_switching_adminbar', 0 );

Tried to use this gist with version 1.0.1.

Got some errors on activation, in this fork I removed them to get the plugin work in a multisite-setup.