Astarion is the best companion we have so far. I kinda dislike his attribute changes that were introduced in the last patch but he is still the best.
It's for the trickster, as I understand it.
The only reason for which I keep Astarion around is that he's the only Rogue we have available other than running as one ourselves and I'll have to admit that I'm quite happy Running my female half-elven ranger knight.I'd have killed him the first time he attacked me if it weren't for that fact
Good news! If you personally don't like some character, that doesn't make him bad. For example, I almost always kill Wyll.