I've googled and read that they should be devastating, but mine have been rubbish. They're concealed, which sounds great, but they're not much good to you sitting where they're deployed and they're no longer hidden when you move them. So getting them close to the enemy generals doesn't seem too easy, which means letting them engage whoever is nearby, and they're don't seem to do as much damage as one of my heavy cavalry units. Are you supposed to get them into melee mode, or just let them throw darts about?
I'm not sure I'd bother with them at all next campaign.