How to reference a file in the plugin directory

Can someone tell me how to best reference the path to a plugin directory (to reference a CSS or js file for example)?

OK… I already have the answer – I just wanted to check I was using the best method

http://codex.wordpress.org/Function_Reference/plugin_basename

This gives:

$x = WP_PLUGIN_URL.'/'.str_replace(basename( __FILE__),"",plugin_basename(__FILE__));

// $x will equal "http://[url-path-to-plugins]/[myplugin]/"

 function writeCSS() {
  echo ( '<link rel="stylesheet" type="text/css" href="'. $x . 'myplugin.css">' );
}
add_action('wp_head', 'writeCSS');

But at the end it also mentions:

WP_PLUGIN_URL – the url of the plugins directory (usually inside wp-content)
WP_PLUGIN_DIR – the server path to the plugins directory

So, why bother with the above function – why not just use the constants?

So… a little bit confused

Any pointers would be appreciated

Thanks

Omar