I have great luck with Adcenter so far. I'm getting about 2.7X what I put in. So if I put in $100 i get back about $272-ish
But what I've found is it all depends on your niche...(duh) lol
Start slow, dont go crazy putting a ton of money in a day. Start little and once you find a good groove (keywords, ads, bids, etc) then you can start "upping it"