I receive the Quora Weekly Digest emails and will tend to open them up and have a skim when I'm looking for something to read. I'd say that roughly 50% of the time they include a topic / response which grabs my attention enough to click through.

I don't engage with the site beyond this, so personally it fills a similar role as scoopinion and other "content recommendation" sites I occasionally hit up to find something to kill a few minutes reading.