I always went single and got owned. I think double and staff are overpowered.
Getting offtopic a bit... :p Oh well.
I use a single, and am a lot better with it than I'm with a staff or dual wield. I don't think that you can really say that any of them are overpowered, it's simply a matter of preferences. Single sabre's hits are slower, but they deal more damage and it's easier to get criticals with it. Staff and DW's attacks are faster, but deal less damage and get less criticals. It's pretty balanced in my opinion.
Also, you wont do well with a single sabre against someone using staff or DW if you simply charge at them, you'll get murdered fast that way. When you use a single sabre, you must be (most of the times) more defensive than offensive, lure the enemy into a trap.
