As I said in the other thread, they're the best light cavalry in the game. They wear plate armor and plate greaves (though every now and then one will wear a coat of plates instead) and about half of them carry crossbows. They have two weaknesses, however: about 1/3rd of them don't wear any helmets at all and they all ride coursers. They butcher spread-out units and they are absolutely excellent units in ground combat, but if you send them into a mob, they get their horses chopped out from under them and their unarmored heads bashed in.
I've also had one beat King Yargolek in 1-on-1 combat despite carrying a saber.
They also aren't too easy to obtain; you have to recruit them from bandits / captured lords or you have to beat peasant parties into submission / raid towns with blunt weapons to get them. In my case, I raised an army of 50 without raiding any villages or attacking any peasants, but still, it's not easy to collect them.