i did not use it before but after this http://www.mattcutts.com/blog/how-to-report-paid-links/ blog posts , i will not use similar services for a long time
I used LinkMarket.net in the past and I did have moderate success with them. I paid the $9.99 a month fee and I got a ton of link exchange requests. My link popularity improved a lot but my PR stayed the same. That's probably because the links that I got were probably buried on link exchange pages (sometimes 4 tiers or more away from the homepage). It helped me in Yahoo but not so much in Google. If you are already getting good traffic then I would stay away but if you're having trouble then I definitely would use it. I still don't understand how they know for sure that someone is selling links. I have links on my homepage going to another site. It looks like I'm selling a link but it's just a link exchange. I needed to entice them to trade links with me because they were an authority site and I had very little leverage for the deal. So I decided to offer them a homepage link.
I use linkmarket but I find it to be a bunch of people are just going through clicking add to cart.. add to cart.. and ends up getting bunch of spam.
Linkmarket has it's drawbacks. There are alot of sites registered there. My site by comparison has not anything close to their membership level, however the software is free and it's really easy to choose link partners with one click. Is there really any way Google or Yahoo can tell a paid link from two sites just sharing a recipriocal link? How would it know? Is it possible that google has gone to the trouble to identify certain known paid link websites and simply listed all the pages as such? (textlinks for example) And if it finds any links coming from any of these sites to your site, then the link gets a different weight than it would receive it were just a recip?