Stay away from that quest dog - he'll annoy the hell out of you, and sometimes cause your death as well by pushing you over constantly - not nice when enjoying the cliff view. Other than that, I don't mind them, but depends on the character. My hunters always use dags, I love dags.
I just gave up with companions all together after a while. They did nothing but get killed by the threats facing me for the most part and getting in my way, so I do a better job of fighting solo.
Uhm, actually, companions can only be killed by yourself...