The Jet Pack is an armor ability in the Halo series. It originally debuted in Halo: Reach and returned in Halo 4. It gives the user the ability to fly and hover for a short time. Which gives a great advantage as it's one of the few abilities than can be used for offense and defense. As you can use the added height for surprises, or use flight to evade any near dangers.
I was a little better using it in Halo 4, but I was never very skilled with the jetpack. I faced many players who could use it like pros. I once faced a trio of players who were like air ninjas that were unstoppable. It all depends on how well you can you aim while in flight. I did find a few players who would just hover and thus become sitting ducks.