I advertise with them but only use the free ad. I find it works perfectly. Almost all my new clients come through it. The rest either google or are word of mouth.
Yell have contacted me a few times trying to get me to use a fee paying ad with them, but as it is working fine as it is, I don't see any reason to pay something like £20 - £50 per month!
Whenever they call I just let them know that I'm still not at the stage to take it further and pay monthly. They have been happy with that and just check that I have all the necessary components to get the ad out to as many people as possible. They have been great really. Can't complain. But I wouldn't pay to advertise with them. I don't find ads in papers or magazines work either. I have my adverts on free websites, I have my own website as well as Facebook and Twitter. I've only paid for adverts twice and neither got me any business.