It's a timely reminder to update this.
So I finished my Quora experiment in mid-March, after giving it a few months.

For me, in the end it wasn't really viable for what I was using it for. It generated a lot of free listings, but the conversion rate was too low from free to paid sales.
For the cost of £418, we generated 1,436 free signups, of which around 40% didn't bother clicking their verification link which probably means they were low-quality spam. Of those that did signup, around 1% converted to a paid option. As we don't chase up leads or do any telesales, then it doesn't make much sense to pay for just free listings.
Compared to Google PPC which is a consistent 9 - 10% conversion rate from free to paid, making the campaign very viable.
But if you are only interested in leads, and will be chasing those down, or you are looking for web traffic generation, then Quora may well be viable for you. It is very straightforward to set up, a lot less complicated than Google PPC, put aside a little budget and try it out.