For assassin, least-used are Khem Val (useless after 50 IMO) and Ashara.
I can tell you - as a darkness assassin, meaning a tank, it drove me nuts to have him around. The last thing I need is another tank as companion, so I put his toggles into damage mode, but he still doesn't excel at that. Being light side and have a creep with such mindless taste for carnage around didn't help much either. His obsession with Tulak Horde didn't make it much better - I'd rather have him locked up in his again, so he can reflect on how everything was better until the end of eternity. It was kinda nice having such a giant monster around - a pretty woman with a her beast. But as soon as I got Andronicus I kinda dumped him.
But effectively he's still the most often used companion around and you won't find another until somewhere mid-progress on Tatooine. And since there is a lot low level players around these days (the most busy planets are Coruscant and Nar Shaddah), you still seem him pretty often. A low level Assassin just doesn't have a choice.