Was a Mercenary until I dinged 50, so I then decided to level up a Jedi Guardian + Sith Juggernaut (one I made because the armor looked better) solely on PvP after level 14 after getting sprint.
Mercenaries are one of the easiest classes to take down from what I've experienced, as long as you have an interrupt and you can close the gap. However, seems like many people get really mad because they get killed by a mercenary or commando while they are trying to kill someone else or while they are doing an objective. Or they weren't paying attention and allowed a Mercenary or Commando to hit all 5 of their tracer missiles/grav shots (which, as Mercenary, lots of people let me do that; I played it like a support class rather than a full on assault class).
Honestly, if you're right up in the face of a mercenary or commando, they can't get their skills off (and part of the time, their skills aren't activating because they aren't facing you). Slow so they can't widen the gap, interrupt, choke or push when they attempt some other channeled ability, run in circles as you attack, and they are basically fodder--especially with all their channeled or cast abilities.