I have tried several traffic services. Have to say that for me it's very hard to measure how effect it was since I don't "sell" anything I can't see uptake in conversions. But I did check my stats and most of the traffic never clicked through to another page on my site, which tells me it either was not the targeted webmaster traffic I thought I was buying or more likely just was not real visitors at all.
For some good low cost traffic from REAL visitors see if you can find any blogads.com or Adbrite -
http://www.adbrite.com/mb/?spid=9423
(I'm the mod so I get to plant referral ids!)
You can go there direct if you don't want to use it.