We initially listed on Capterra as it seemed to rank very highly in google (position 1 or 2 always) for the key google search terms for our product. When your own product is on page 10 it seemed like a good way to get a bit noticed.
To get people to post reviews we basically just asked them, contacting our known good users. This is something we probably need to do more of. I know a lot of companies offer incentives (Amazon vouchers) to get people to post reviews and Capterra now ask if you do this.
Capterra do have a PPC offering as well and we used it around 4 years ago for a while and got great return on investment. However is it got more popular and the PPC prices went up we stopped doing it. I guess you just need to see how much it is for your category.
I would say of all the product review sites Capterra has easily driven the most traffic for us, it’s the only one we really bother maintaining.