For software, I can say with confidence that Capterra has more traffic than any other directory that I know of.
We market B2B software and Capterra's directory rivals the traffic of a targeted Google Adwords campaign for the same. The quality is good, too. We measure conversions through to leads and sales and it works well.
I recommend them, but like anything you must: * have relevant landing pages with good offers * take the time to look through their categories - you can be in multiple ones, and you should be in all of the ones that are relevant for you. * bid to be in the top 3 spots to get any decent traffic. In the top 3 you should find lots of traffic.
Depends on the software/industry...and probably a host of other factors - are you looking for leads? shopping cart purchases? is that standard for your industry? etc. :)
I regularly see Capterra leads come through for B2B software that cost 2-3x Google AdWord's cost per lead...with very little difference in sales conversion rate.
We're talking about software that costs $20k+ to implement and license...so the Capterra leads are easily worth it given the size of the market for this particular software.
So I would say to be conservative, you may want to calculate 2 or 3 times your AdWords cost per lead if you're pursuing leads and if you already use AdWords...Sorry I don't have a really good answer for direct sale/ecommerce purchase conversion rates from Capterra.