Well, i've played my Ninja/FF MM up to level 26 and I find that it's one of the most enjoyable experiences i've had with CoH/CoV.
The Ninja's are great all-rounders, especially when you get your Oni as it has some great ranged attacks. My usual tactics mean buffing each pet with Train, shields and invisibility, then adding my bubble and Assault + Tactics. While this does take me two full bars of End to do, it means my minions are pretty much undefeatable. I solo Ruthless missions, I just recently killed Sea Witch & Ballista-1 alone without dying and I pretty much kick a lot of [censored].