You would expect this to make them good buddies, but its anything but. Any hint of conversation and they immediately argue. Good thing they both like to stick to the task at hand, unlike some whingers. In fact, HK-47 seemed to just sit around and watch his master torture innocent people. Well, he is an assassin droid.
What's remarkable is that HK-47, being very strong and sturdy coupled with continuous repair from T3-M4, actually managed to survive all the wars he has been in and lived up to and beyond the Galactic Civil War. What an awesome guy!