One important thing to keep in mind is that the number of clicks you get with any mailer are a lot less important then the number of conversions (signups or sales) you get with them.
A lot of mailers that give you a high number of clicks are incentiviced mailers and the majority of people just click to earn credits are not interested in what you are offering.
At ViralURL.com & ViralHosts.com we don't inventivice clicks to give you a true way to e.g. test mailings before you use them for paid solo ad mailings.
Colin made a video where he actually tested the conversions rates with several mailers...
What other mailers do you know that don't inventivice clicks?