Guild Wars 2

Are thieves just generally useless in dungeons?
I mean other than sitting in the back with a shortbow are there any viable builds?

I honestly dont think its weak. I just feel you have to play it in a certain way to make it really good

Here is my current build
http://gw2.luna-atra.fr/skills_tool/

Also how do you build full crit?

I have a lot of fun playing thief Sword/Dagger as i like dipping in and out and just like the overall feel of the weapons. But i feel the damage is lackluster or I am playing it wrong both in PVE and sPVP.
How should Sword/Dagger be played? Is it viable at all? What kind of builds should i use?