I am a lvl 35 and what i understand, lvling with Assassination or combat is the way to go. But I may have discovered something to fit my playstyle and maybe some others too. Its hybrid, a cross between Assassination and SUB. It distributes the same amount of steady power while your in combat. Your backstab is the same as having the assassination build, but your ambush is REALLY big. You also have the benefits of the sub tree. Check out my build and tell me what yall think. I own in pvp and i do the same and better when doing instances.
http://www.wowarmory.com/character-sheet.xml?r=Spirestone&n=Silentest