I have found them to be totally useless. After about the twelfth time reloading my game because RL-3 couldn't keep himself from being destroyed, and the tenth time being given away by Star Paladin Cross in Murder Pass (and thus being ambushed by a bunch of Super Mutants, yet again), I have thrown in the companion towel. I'm keeping Fawkes as pest control in my Megaton shanty because I honestly can't be bothered with making sure he stays alive and/or doesn't blow my cover.
I'm a bit of a wimp -- I don't like to be surprised and so my character(s) always creep around and try to peg off the nasties from a distance, strategy that gets tossed out the window the moment someone else makes some kind of battle cry and rushes right in.
I kind of miss using RL-3 as my personal pack brahmin, though...