Launchpad can be used as additional shortcut keys for Reaper's actions (though if you don't write on the buttons or attach some stickers on them, it might be tough to remember what button is for what action...)
I've myself done some experiments with Justin's new midi2osc tool, various things for controlling effects or Reaper's mixer can be implemented by generating OSC messages in response to the keys pressed on the Launchpad. This requires writing some custom script code, though.
About midi2osc :
http://www.1014.org/?article=511