To everyone defending the class, you should probably make sure you watch the video... it really does look ridiculous. The reason sneak attacks are supposed to be OP is because they're hard to perform, but that's clearly not the case if you go the route this guy did. I've got no problem with 15x damage if it was actually a struggle sneaking up on the enemy in the first place.
So right now I'm planning on ignoring the left side of the sneak tree and not using invisibility potions at all. Is it alright to put 5/5 on the Stealth perk though? 40% harder to detect shouldn't be too OP, right?
Oh, and I freakin' love Blackthorne!
I've leveled my thief quite a bit and am currently level 66, so I have been putting a few perks into the One-Handed weapon tree. I've maxed out this tree for duel wield swords, and even on Master difficulty the game is now a breeze. Who needs to sneak when I can just walk up to any monster/boss and simply hold both triggers and kill them instantly with a power attack? Every class is "overpowered" if you know how to use it correctly. Yes, I can sneak up to a dragons face in FULL DAEDRIC(who needs light armour) with my enchanted assassin(sneak+OHW damage) daedric set + muffle spell, but why bother? I can go berserk warrior mode with my duel wields and wreck them without sneak.