I've had luck with SM. Leads are very reasonable and they are good about giving credit if something is wrong with the lead.
On a lot of the boards guys complain about SM giving them bum leads or duplicate leads and so on. But they've never done it to me, I only requested credit twice when the lead I called apparently was only curious and not serious and I didn't even end up doing an estimate. SM gave me credit no problem, even if they didn't give me credit its still only $12 a lead compared to a postcard lead which is close to $100 acquisition cost.
This is what most guys don't get, they'll pay the yellow pages $500 a month for an ad that brings them 2-3 jobs a month and then complain about SM costing $12-$15 per lead.