My mage brings Erik the Slayer with her everywhere. She's a squishy mage, and most of the time he makes for a very good damage sponge. My usual strategy in tough battles now is to flank the enemy and get on with the killing while they're distracted trying to turn Erik into paste. He's been pretty good at not setting off traps in dungeons, too. Or if he does, he usually only hurts himself instead of me, which is appreciated.

(Plus, his "earnest, vaguely naive farmboy" character amuses the heck out of me most of the time. I always know to save my game in a dungeon and to prep for a boss fight when he blurts out with, "I don't think I like where this is going!")
Meeko will tag along sometimes too, but I never bring him into dungeons with me. He has a tendency to get in between me and my spell targets in enclosed spaces, which never ends well.
I do miss a lot of the options (like telling them to equip clothing or armor for RP purposes, or to use ranged vs. melee, etc.) and story/dialogue you'd get from companion mods in Morrowind or Oblivion, but...eh. I was anticipating that the vanilla game companions would be hollow shells compared to, say, Julan. I'm just happy I've found one I like enough to put up with hearing the same lines of dialogue repeated over and over.