I was reading some Whedonesque reactions to Ausellio's reaction to Dollhouse, and I think I may have put my finger on what worked for me about Eliza in Dollhouse and what didn't.
Watching her, I believe that *she* (or Echo) believes she's each of these people she's imprinted with. She plays each character with complete conviction that this is who she is, and no hint of uncertainty in that respect.
However, I'm not sure she's good at being each of these people.
Does that make sense? I believe she believes she's Eleanor Penn. But that doesn't necessarily mean she's a convincing Eleanor Penn-as-hostage-negotiator. That might be my disconnect between thinking she was better than I expected yet not entirely disagreeing with my boyfriend that she wasn't that convincing in all the different roles?
And that may be my problem with her Echo-as-blank-slate in between character. She's just so vague, there's no real sense of self there. Which is the point, I'm sure, but it's not as convincing...