I played around with the Thief for about 3 hrs, i didnt really find a build i could recommend as i was swapping around abilities as much as i could just so i could test everything as much as possible. Ill do a small write up on my findings with the Thief soon, it wont be a big 1 as i had limited time but i will list the abilities that did stand out to me as being fairly good.
I wish i had more time to test more as i only tried the SC based pvp and nothing else. With the gear i was given with the lvl 80 template i did fairly well in pvp considering i was fumbling my button setup alot and messing up my rotation ( swapping in and out diff abilities every match does that ). I feel in the hands of a skilled player the thief will be bloody hard to beat 1 vs 1 and hard to kill even if you come across a thief with superior numbers. The amount of tools you have to get away and sneak back into the shadows was impressive. Anyway ill do a small write up in the next couple days.
Edit: I will hold off on a review of the the Theif until after the next beta weekend so i have a better grasp on all the skills, i would like to experience some WvWvW with the Theif and have a better undersatnding of the class in all aspects of the game.