I play a guardian and personally find Rusk to be one of the more useful companions. I generally leap into a group, area taunt, sweep and let his aoes mow the rest down. He has more aoe attacks than Kira does, he is ranged so he's not getting hit all that often and on the one occasion that I noticed he actually pulled a mob on his own, he has the heavy armor to take the hits until I could leap/guardian leap back and help him. That said, I find I can't make much use of Kira, so our play styles may just be different enough to make the differences between Kira and Rusk meaningful to us. I'm not sure why you think Talah would be any different than Rusk though. Sure she's cool, but I image she'd play pretty much like the Sith Warrior's Vette which means she'd be a lot like Rusk.