I played in one session of a friend's Return to the Temple of Elemental Evil, just as they were trying to get inside the front door of the crater (I think).
Anyway, I was playing Eljay, a gnome sorcerer with a dachsund familiar (stats roughly equivalent to a rat) named Puckers. We got inside, and were attacked by a group of baddies, including an evil cleric who struck me blind with a spell as the battle was winding down.
I was nearly helpless, unable to target spells, unable to move to safety. All the people on my side were engaged in other combat, and most of them were near death. One wounded enemy warrior managed to slip out of the melee, and he headed over to me to finish me off.
I told the DM that Puckers was going to attack the warrior. What followed was a five minute argument about how stupid it was to try to have a dachsund attack an armed warrior, and about how the dog wouldn't endanger itself. I am firmly in the position that the player should be allowed to play his familiar as he sees fit, I ended up cussing at the DM's girlfriend (the most vocal of my opponents) to tell her to shut up, and I told the DM that my familiar was a dog, a gnome's best friend, and it was willing to risk its life to save mine, like any good dog.
I rolled the die, and got a natural 20, and confirmed the critical.
Puckers growled, and as the enemy stepped in to swing, my heroic familiar leapt into the air, bit into the warrior's throat, and crushed his windpipe. The warrior's attack of opportunity missed, and it staggered back, falling to the ground, unconscious.
I, for understandable reasons, left the game after that one session, with Puckers as my new seeing eye dog.