This past weekend I got back into my Mercenary. I was able to push him through Chapter 2 fairly quickly and reach 45 so I could get Heatseeker Missile.
I haven't had much issue thus far in PvP (or PvE) with the class, certainly not as much as people have indicated. Now, bear in mind I'm speaking from a pre-55 standpoint, but the class has a very diverse and useful toolkit. It allows for a very flexible play style, and the only thing I can really note that seems to make or break my success with this class is how I position myself in the battlefield.
I recorded a few matches over the weekend, but I only uploaded a Huttball game due specifically because of positioning. A lot of the matches I was able to just gun down opponents without much effort, and in review it wasn't quite as interesting as this game. I uploaded it to my YouTube channel if you want to have a look. http://www.youtube.com/watch?v=C4mvQGcNjQM