I usually recommend going with the bigger engines first and creating accounts that work there before even trying the smaller sites. I don't use the small PPC engines much anymore, but I think when picking them it depends on vertical. with small players one good publisher partner or one shoddy one can make or break the ROI of a campaign. I have been a searchfeed contextual publishing partner that siphoned off great traffic from Google, but I also have bought traffic from some of these types of engines to a bounce rate well above 90% http://www.ppcqueen.org/6/ reviewed 7search recently.