I've used them from time to time and they can definitely pull in leads. They can come in at the oddest times, too. For example, I left an ad for my social media lessons up on OLX several months ago. I recently got an enquiry as a result of that, which converted to a sale.
When placing ads you should always be mindful of the keywords you're using, particularly in the ad titles. This is because a lot of the big free classifieds sites are constantly being indexed by Google and can rank highly for various terms. So you should think about what people will be typing into search engines most frequently to find the kind of goods and services you're offering.
Hunches can be pretty effective, and you can also use free keyword suggestion tool to generate ideas. If your ad is local then always remember to use location names. (Suburb names are probably too specific in a lot of cases, but city names are usually worth including.) Once you've decided on a good phrase that includes two or three such words, put them in the title of your ad.
For example, I recently placed an ad for this site. I knew that the search "learn social media Perth" was reasonably frequent because it appeared in Google's drop down suggestion box. So when placing my ad on Gumtree (one of the biggest and best Aussie classifieds sites) I included those keywords.
Sure enough, it ended up at the top of page one for it. As of this writing, I haven't had a call coming directly from this search or something like it. But it's still a very good sign.
I will of course try different combinations on other classifieds sites to see if I can rank for some other keywords searches and therefore get more exposure.
Needless to say, search engines aren't the only way that people find these ads. But many certainly do, and given that they trust Google so much, it helps if you can get as much of this kind of traffic as possible.