I think all bar the Culexus and Vindicare have a role to play.
Eversor: A straightforward killing machine, equally adept at ripping apart T4 infantry (has a poisioned power weapon essentially, hence re-rolls to wound on T4 or less) or killing MC's (his I5 and large number of attacks mean even Greater Daemons should be worried). Outflank has breathed new life into him, it's caught more than one player off guard (he has a 12" charge range) when he's arrived on their flank and eaten a Devastator squad etc.
Callidus: More a precision weapon, she's really only suited to picking on T4 characters/heavy infantry. Her real bonus is 'A Word In Your Ear', which can be game-changing. Essentially, the Callidus lets you pick an enemy infantry unit and kill them with total impunity. She is very vulnerable to ganking in combat (unlike Eversor she doesnt cause enough wounds to win combat decisively), so pick your target carefully.
Death-Cult: At the same price as a Terminator, you ditch the 2+ armour and any ranged attacks for Infiltrate, more attacks and I5. Used correctly (ie grouped together), they make the Eversor and Callidus much more dangerous. Infiltrating them en-masse at a set target is both effective and hilarious; each Assassin has to be shot individually (yes, that does suck in 'Annhilation', but my gaming group largely plays objective missions for that precise reasons ie KP's are retarded), and each Assassins pumps out 4 x S4 power weapon attacks on the charge. They also each resolve Sweeping Advance seperately.
Do not Outflank them, unless you've got some powerful reserve bonuses (IG Astropaths, Varro Tigerious etc). Their key to success is as support to an Eversor or Callidus. They can let the Death-Cult clear out the chaff (ie retinues, attached squad, big infantry units), while he/she goes for the character or MC.