I was just wondering the same thing actually - I've played two Vanguards, a Commando and a Pyrotech over the past year and created a new BH last night. Since my Pyro was on another server it was a 50:50 choice between the two. Given that it had been 8 months since I played my Commando, and I had been playing a Vanguard a lot lately, I decided to pick Mercenary.
I've spent all my time since then wondering if that was the right choice. PvE seems ok, PvP average but more to do with DPS output rather than interrupts: I wasn't against any decent opponents, perhaps got interrupted 3-4 times during the two WZs I played, but still managed just average DPS.
I've played every class to 30-50 (signature's very out of date) and Merc is probably the only one I've had real doubts about, but I think I'll stick with it because, as someone in another thread pointed out, we've hit rock bottom. The class can only get better.