I've only played the Tech Specialist and Mercenary classes so far. Smuggler hasn't interested me, figure I should do a run through with them eventually. But I have to agree with what everyone else says here: Mercenary makes combat a joke. They literally can smash their way through all the encounters by virtue of their ludicrous damage output (200% damage for one attack, 10% chance of crit, raw 20% to kinetic damage), making advanced tactics unnecessary. They shrug off most damage, due to innate resistance and stun/paralysis recovery, which is boosted by whatever armor they happen to be wearing. Group fights are dealt with by Cleave, or are blasted down by amplifying consecutive ranged attacks. They don't need to stand up if knocked down, since their reduction in stats can be ignored by your natural toughness and damage multipliers. And once per battle, just regenerate most of your health and energy for free! The only thing that might screw them over is grapple attacks, and the cum-cow perk line now gives you a way to auto-escape grapples, something only Tech Specialists used to have with Static Burst. If you don't mind being a bimbo Merc, you can curb-stomp almost everything in your path with no worries. They are the easy mode class.
Having said all this, I don't think the Mercenary class is bad. Hell, I'd be lying if I said it wasn't fun to wreck everything that tries to jump you. And lust inducing enemies can still get you if you're not careful, so you're not totally invincible. It just feels that they other classes struggle quite a bit, and have to optimize their gear choices if they want to be viable, while the Mercs can just roll whatever they want and still probably come out on top. Frustrating, but I main Tech anyway, so I guess I like the pain.