One of our play group used to use an assassin with his ultrasmurfs, he took an inquisitor and the psychic assassin - he used it to kill our two Eldar players and if not against them just as a nasty lurking surprise.
I am not sure about using them with Chaos, I wouldn't think so however.
Yeah, Chaos already has their own psykers, called Sorcerers.
As for the assassins themselves, the Callidus assassin seems to get used the most, and was even used in the Imperial Guard list that won in Chicago's Ard Boyz finals this year.
As for the others, they all seem to be equally popular, although not used too much in IG or Space Marine lists, because of the trouble it takes to add one. The only assassins that don't seem to be used are the Death Cult assassins. I've only ever seen them used in apocalypse games.
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