I mean think about it. Yeh you get a few perks while alive - but you're damned to eternity to guard that dingy cave when you're dead. Doesn't sound like a good deal to me. I was dissapointed that there were no conversation options to say no, I don't want to become one. Hell if you yes i'm ready to Karliah and follow her to the circle SHE SAYS THE OATH FOR YOU!! I mean wtf? I wanted to tell Nocturnal no to her erm face..well ball thing...but no Karliah practically sells my soul so she can get her revenge and I don't get to say anything about it?? I immediately whipped out my swords and started hacking those two up, only to find that they can't die...damn it. Very pissed about that. Loaded up an earlier save, threw the nocturnal armour on the floor and left, determined never to return. Guess I can't complete the thieves' guild quest line eh? Bit daft you have to sell your soul to catch that idiot Mercer.