It seems to work well for some people and not for others - no idea why, whether it's area, placing/size of the advert or the actual advert used.
We do quite well from YP but it has definately gone down over the years - it used to be well worth our while (paid over a thousand a year for a couple of areas and got a lot more back) and now it's OK but Yell.com we find to be very good - I.E. the two together are still worth it.
I think that possibly you need to have a BIG advert - as in 1/4 page or more for it to get that many calls, and then you have to spend more so it is worrying!
Anyway, like I said - works differently for different people. I would probaby say use yell.com but not YP.
Good luck!