I've become an avid BorrowBox user, partly because I can't focus my eyes on a book by evening, and partly because it helps me get to sleep.
I know a narrator is good when I suddenly realize, part way through a book, that I haven't actually given the narration a thought!
(It's unfortunate that I (we?) notice what's not good so much more quickly than what is good.)
The very last book I listened to included several working-class characters from Sheffield.
Unfortunately, the narrator had no idea how to pronounce the glottal 't' that was ever-present in the dialogue.
Each time it was needed, she was adding what sounded like a whole extra word: to. This resulted in sentences such as, "She'll start work in to factory^ ", and "He's gone down to to pub to talk to to landlord".
As a native northerner, I found this very distracting, as I reflected each time on what she should have said.
The book I'm listening to at the moment has a narrator whose French characters mispronounce common French words such as Monsieur. OK, I only notice those because I can speak the language, so I'm probably being a bit unreasonable, and she is otherwise a decent narrator.
There are some great narrators out there though - I just wish that the technology was there to enable us to select whichever one we wanted for every book that we download. 