Depending on the partner they will be more or less usefull, Joel is usefull, Dina isn't. Most of the time your partner will only kill the last surviving enemy in the story. And the AI seems to vary greatly with the difficulty. On the lowest difficult their route is easy to predict, easy to avoid and you can get very near even going from the front and still stealth kill, while on hardest they will spot you very easily.
I think I'm on the default difficulty, don't remember if I changed it. They spot me quite easily though, definitely can't sneak up from the front. She could at least share some of her infinite supply of bullets! I suck at the dodge mechanic as well which doesn't help. Yet why will the enemy not go after her, so I can then attack it from the back. It's very immersion braking when your partner stands there as an invisible (to the enemy) turret that can't hit. Just wait outside if its going to be like that :)
Anyway after that I figured out how to swap guns (that's where all the ammo went I picked up). Not sure why the game doesn't automate that or prompt at least like it keeps doing for jumping and crouching lol. Hit and miss, basically the same as combat in the first one. Some encounters are great.