Snarky narrators

I've been musing on the qualities of the snarky narrator, and wondering: when is he funny, and when is he annoying?

I think it has to do with vulnerability. A character who is always on top of things, who always has the perfect comeback, needs no sympathy or emotional investment from the reader. Also, he becomes the player of a one-note symphony. Nobody can be "on" all the time--we need the variety of the occasional sad or pensive moment, the flash of fear or anger, the moment when the character becomes utterly befuddled.

But those are just my initial thoughts. What do you think?