We do advertising on the Canadian Yellowpages site and it's affiliates. It seems to send pretty good traffic our way (although we're modding our website a bit to try and beef up conversions).

We're listed as 'National Leaders' for a few search terms...meaning we come up in the top 5 for the given terms. They CAN guarantee the results, as they control them. It all comes down to how much you're will to pay for it.

Just make sure you've focused your keyword choices so you get relevant traffic.