I've always like the Night Lords. Their background fluff and mode of fighting has always appealed to me. However GW never really did them right. Rather than terrifying the upgrade kits released 10 years ago gave them "bunny ears" - cue jokes about stealth impossibility - and so I never got any.
I think this might now change. Forgeworld have released Sevetar, the First Captain of the Night Lords and what a great figure.
The model comes on a display base with decapitated White Scar (Hurrah). I've also seen the Konrad Curze model and that only adds to the appeal. So I may add a unit of Night Lords as an ally to the Death Guard in the very near future.