Personally, I did pick up enough work from Yell to cover the ad cost and a bit more besides. A few years on I am still doing one of those jobs - it's worth over £500 a year to me. However, it got me inundated with cold callers trying to sell me things - all of which were totally irrelevant for a sole trader. I even went through a time when I stopped having my mobile on divert from the landline. Not only was it time consuming receiving such calls when trying to work, it was costing me money for the calls too (my landline to my mobile).
Overall, I have made a profit on Yelladvertising. I stopped it the day they phoned me yet again about renewing. They find it hard to take no for an answer so after a number of calls, I just told him to go away (paraphrased). That was the last I heard.