I've been using it exclusively :S. I am a thief. Would it make sense for me to use one of the lighter swords with greater reach?
The dagger's short range makes it challenging, but it's so fast and uses very little stamina that once I get a hit off I can slash most enemies, even tough ones, to death. I just murdered Havel in an extremely long poke fest.
Bandit's Knife is really good, it's just... bad in pvp due to its non existent reach and pressure. Spamming R1 means you can parried into oblivion. But it has bleed, the backstab damage is good, no hate.
Problem of course is that the Rapier and the Estoc do more damage on backstabs due to their hidden piercing property that lets them go through armor better. They also have better range.