Did you miss me?
Ah, the honey-voiced dreamy one! Don't get me started. Fed into your battle suit at an early stage of the proceedings, Cortana offers the occasional bit of advice and helps drive the storyline in the cutscenes. Handy and cute! Every cyborg should have one.
Just remember who's boss
Tactically however, there's not much to say about Cortana, except to point out that you don't have to do what she suggests. There are times when her tactical suggestions are poor. In addition, she tends to want to keep you on the straight and narrow, and by ignoring her you may have some unusual fun. Indeed, it was only by ignoring her warning not to leave the area that I bumped into the rockslide megabattle on returning later, which has been keeping me fairly busy ever since! And at the start of level 9 she insists that you jump into the uninviting pool of goo below, but by ignoring her you find yourself in the excellent Flood onslaught challenge. So by all means listen, but just remember who's boss.