Duelist/Guardian is amazing!.. For the beta. After which it's usefulness starts to scale down and be replace with just having better gear. You'll find more use out of it with other classes.
It works very nicely with Katar in PvE and somewhat in PvP. Example gameplay:
Wizard is also a somewhat interesting sub, for PvP only, due to the party buff and dash.
Gunslinger is nice with it's traps in addition to the Duelist DoT's.
Sorcerer can be somewhat nice with it's CC skills to help Duelist out.
It's all really upto you, how you play your class, and how you build it out to be.