I played Powertech to lvl 20. Deleted it to start Mercenary because they seemed to do omgwtf damage and Powertech seemed weak. I got my Mercenary to lvl 28, while damage was good I got utterly bored with the missile spam. Now I started new Powetech and are very pleased with the Pyro tree understanding how the class and the tree works and what you are supposed to do to get big damage rolling.
IMO
All Mercenary can do is sit there and shoot missiles, hoping that they hit their target. Powertech has better utility kit and what I like them most, can adapt to situation having unique advantage of being able to have range advanatage over any class. Facing an assassin or Marauder? Kite them with your ranged nukes and dots and snare. Facing Mercenary or Sorcerer? Grapple them and go on their face with your interrupts.