The Night Haunter was always the one Primarch whose model I'd seriously need to pick up, if it came out. Thus my excuses for not getting into the Horus Heresy books/figures dwindle further...
He's not as beefy or chunky as his brothers, but he more than makes up for it with his ability to straight out murder just about anything he comes across. His biggest thing though is causing Fear with a -3 penalty on enemy morale. He gives all Night Lord models in the core detachment Fear as well, can always say that the first turn of the game is at night and of course, he's got both stealth and shrouded, is jump infantry and has all the usual Primarch bonuses. He's the guy you want to stick into an assault squad, throw at any enemy infantry and watch it go on a happy little rampage.