I love the jetpack, it is SUPER OP, in the case where you can just hit and run enemies, so you dice them up all while getting out unscathed. I also really love the bow, but my aim is terrible and half the time I am forced to use the sword because my arm got lopped off. I still really love the deflection though, cause it has saved my life and gotten me kills more times than I can count. But that's just my two-sense on the subject :P
The jetpack isn't OP.
Neither is any of the weapons.
The Jetpack is made for fast manouvers. If you are quick at moving and have good reflexes, and know how to use it, then yes, the jetpack is OP. Same as all of the other upgrades.
The bow is OP is you can aim properly, simple as that.
Deflection isn't that good. It leaves you vulnerable in later levels, as archers are positioned in better and better places.
The deflection is easy to use, and better for earlier stages. That's why lots of people are flocking towards it.
Though I will agree, it can be useful in lots of situations.
(Then again, I'm no master of this game myself. I just got it 3 days ago!)